Handbook Corrosion (1992) WW Part 13 ppt

"Protection of Austenitic Stainless Steel from Polythionic Acid Stress Corrosion Cracking During Shutdown of Refinery Equipment," RP-01-70, National Association of Corrosion Engineers, 1

The most common fiberglass-reinforced plastic curing system uses methyl ethyl ketone-peroxide (MEKP) with a containing promoter an obvious source of cobalt ions The resin manufacturers provide mixed recommendations regarding alternative curing systems, some indicating that increased fabrication difficulty more than offsets the benefit of eliminating the cobalt The quality of fabrication is a most important factor in successful applications of fiberglass-reinforced plastic and should not be compromised Most resin suppliers recommend synthetic veils Synthetic materials should certainly be used for strongly alkaline solutions Postcuring is also recommended and provides a more resistant fabrication Full curing of all secondary joints is most important Finally, thixotropic agents should not be added to resin systems, especially when used in surfaces exposed to hypochlorite

cobalt-Glass-flake reinforced spray linings are also used in hypochlorite services The problems encountered indicate that these applications should be limited to low-temperature services or applied where the environment is not overly aggressive to the substrate

Elastomeric lining is common in a wide variety of hypochlorite applications a result of typically low temperatures Probably the most frequently used lining material is chlorobutyl, because of its good resistance coupled with moderate cost Other sheet lining materials with good resistance include EPDM and chlorosulfonated polyethylene Natural rubber also finds some useful low-temperature applications Other elastomeric applications might use the fluoroelastomers, which are good in any hypochlorite service, or neoprene, which has limited range

Glass is unaffected by hypochlorite within a moderate range of alkalinity and temperature Glass-tubed heat exchangers have been used to cool soda bleach during manufacture Vitrified clay pipe and other ceramic materials show excellent resistance to hypochlorite Under certain conditions, concrete is resistant and has been used for manufacturing and storage tanks

Calcium Hypochlorite

Although produced as a solid, the reactions of Ca(OCl)2 in solution are very similar to those of NaOCl In general, the recommended temperature levels for Ca(OCl)2 are slightly higher, probably owing to the higher decomposition temperature Like NaOCl, the calcium product is unstable at lower pH, but it can be concentrated to a higher degree The corrosion rates for some metals are shown in Table 49

Table 49 Corrosion of metals and alloys in Ca(OCl) 2

204-day test in 18-20% Ca(OCl)2 at 20-24 °C (70-75 °F)

Corrosion rate Material

mm/yr mils/yr Pitting

Zirconium 0.025 1(a) none

Hastelloy C <0.0025 <0.1 none

(a) Severe attack under spacer

Plastics and elastomerics can usually be used in Ca(OCl)2 to slightly higher concentrations and temperatures than in NaOCl Virtually the same applications are served by these materials in both products

The solid Ca(OCl)2 product is typically packed in polyethylene or polyethylene-lined containers Epoxy-phenolic lining performs effectively for trucks and railcars It is critical to keep the product dry and away from organic fluids Aluminum

is sometimes used in handling solid Ca(OCl)2 because any corrosion residue does not discolor the product

Corrosion by Ammonia

A.S Krisher, ASK Associates

Anhydrous ammonia, a major commercial chemical, is used in the manufacture of fertilizers, HNO3, acrylonitrile, and other products Except for a sensitivity to SCC, carbon steel is fully acceptable in NH3 service Stress-corrosion cracking

of carbon steel NH3 storage vessels was first observed in the early 1950s In most cases, the developing cracks have been detected by inspection before leakage or rupture However, there have been a few catastrophic failures For example, in France in 1968, a tanker ruptured, killing 5 people A second case was in South Africa, where a large tank failed in 1973 with 22 fatalities

Ammonia is stored under three conditions It can be stored by cooling it to a low enough temperature, (-34 °C, or -29 °F)

to maintain it in the liquid state at atmospheric pressure This method is frequently described as cryogenic storage A second approach is to contain the ammonia under sufficient pressure (about 2070 kPa, or 300 psig) to maintain the ammonia in the liquid phase at ambient temperature Cylindrical pressure vessels are often used for fairly small quantities Spherical pressure vessels are used for larger quantities The third condition involves some degree of refrigeration combined with pressurization This is termed semirefrigerated storage

Most cases of SCC have occurred in ambient-temperature pressurized storage vessels, for the most part in spheres A few problems have been observed in semirefrigrated storage There have been no documented cases of SCC in cryogenic storage vessels When SCC does occur, cracks are primary transgranular and progress at a relatively slow rate compared

to other SCC phenomena

Laboratory Studies

One investigation using statically loaded tuning fork type specimens and tensile bars showed that NH3 SCC is accelerated

by cold work, welding, applied stresses, and the use of higher-strength steels It was found that air contamination promotes SCC and that water in amounts greater than 0.1% inhibits cracking (Ref 217)

Other experiments using slow strain rate tests and a low-alloy steel also found that air contamination promoted SCC and that water at a level greater than 0.09% was an effective inhibitor Electrochemical studies showed that the SCC involves

an anodic chemical process (Ref 218, 219)

Field tests were conducted using specimens stressed by residual stresses from welding (Ref 220) Results indicated that high-strength steels fail more rapidly than low-carbon steel and that hard welds (welds that are harder than the base material) tend to accelerate cracking Thermal stress relieving was also found to be effective in preventing SCC

Another investigation using low-alloy steels and slow strain rate test methods produced SCC at temperatures as low as 0

°C (32 °F) Again, air contamination and low water content promoted SCC (Ref 221)

Results of an industry-sponsored technical investigation that used both slow strain rate tests and fracture mechanics type specimens are documented in Ref 222 It was found that oxygen levels greater than 5 ppm are required for SCC Levels as low as 1 ppm caused cracking if carbon dioxide was also present and water was absent This work also suggested that hydrazine (NH2·NH2), ammonium carbonate [(NH4)2CO3], and ammonium bicarbonate (NH4HCO3) might be inhibitors The fracture mechanics test methods were not successful, possibly because of the slow rate of cracking

Other tests using low-alloy steel and the slow strain rate test confirmed again that oxygen as a contaminant is damaging, with indications that levels as low a 0.01 ppm might be sufficient to cause the cracking, at least in low-alloy steel (Fig 85) Nitrogen also appeared to be a cracking accelerator in combination with oxygen The lower limit of the required water content to inhibit cracking was found to be about 0.08 wt% (Fig 86) This work showed NH2·NH2 to be an effective inhibitor at 0.025 wt% for a contamination level of 200 ppm O2

Fig 85 Effect of oxygen content on apparent ductility observed in slow strain rate tests of low-alloy steel in

liquid NH 3 Source: Ref 223

Fig 86 Effect of water content on apparent ductility observed in slow strain rate tests of low-alloy steel in

liquid NH3 Oxygen content was 200 ppm, added as air Source: Ref 223

This body of laboratory work (seven studies over a period of 19 years by six different investigators using three different methods in four different countries) is impressive in its consistency All of the studies showed that the primary causes of the cracking are high stresses and air contamination Nitrogen and carbon dioxide were suggested by separate investigators as promoting SCC Cracking is accelerated by the use of high-strength steels, the presence of hard welds, and air contamination The cracking mechanism can be inhibited by water above about 0.1% Thermal stress relief, if done properly, reduces stress below the critical level

Field Experiences

Some reports suggest that water is not always an effective inhibitor, especially when water is added after SCC is detected The significance of these reports is clouded by a lack of evidence that adequate control systems were used to ensure that a sufficient level of water was maintained The research studies discussed previously do not address the effectiveness of water addition in slowing the growth of pre-existing cracks

There is also a problem area with the vapor phase of NH3 tanks Water is considerably less volatile than NH3, resulting in

a lower water content in the vapor phase than in the liquid If NH3 vapor condenses on the wall of the vessel, the water content will probably be inadequate for inhibition, and SCC in the vapor phase is possible

There are also reports of recracking of vessels that cracked, were repaired, and then were stress relieved It is extremely difficult to repair vessels that have suffered SCC There are many cracks in the equipment, including some of submicroscopic, size It is extremely difficult to prevent these cracks from propagating later Stress relief of a vessel that has suffered SCC is also very likely to be unsuccessful The very small cracks are contaminated to some degree When this metal is subjected to the stress-relief thermal cycle, such phenomena a nitriding and carburizing may occur and promote further cracking

Practical Operating Guidance

It is apparent that SCC of carbon and low-alloy steel NH3 storage vessels can be a problem if proper procedures in design, fabrication, operation, inspection, and maintenance are neglected If the degree of such neglect is large enough, catastrophic failure is possible However, it is also apparent that application of proper procedures will ensure satisfactory long-term storage Reference 224 discusses such practices General recommendations for design, fabrication, operation, and inspection and maintenance practices are presented below

Design and Fabrication. Normal design methods used for vessels to contain hazardous fluids should be followed, including all requirements of governing codes and agencies Design should also be reviewed using fracture mechanics concepts to assess the risk of brittle fracture Fabrication should be carefully inspected by a properly qualified engineering representing the end user

A low-strength (specified tensile strength not exceeding 483 MPa, or 70 ksi) grade of carbon steel should be used The hardness of welds should be specified to be 255 HB maximum, and the weld hardness should be checked in the field

Postweld heat treatment (stress relief) at 595 °C (1100 °F) minimum should be specified for all pressure vessels The lower temperature/longer time alternatives for such treatment allowed in some codes are less effective in reducing residual stress levels

Operating practices should minimize air contamination Water content should be maintained at 0.2% minimum if

water is not objectionable to the user of the NH3 Water (and, if feasible, oxygen) content should be checked by routine sampling and analysis

Inspection and Maintenance. All tanks in NH3 storage service should be carefully inspected on a routine basis A new tank should be carefully inspected by the wet fluorescent magnetic-particle method after 1 to 2 years service If no cracks are found, a somewhat longer inspection interval may be appropriate If cracks are found, their severity should be assessed and appropriate actions taken These actions may involve simply recording location and size of cracks, grinding out the cracked areas, or grinding out and rewelding the cracked areas Figure 87 shows guidelines regarding modifications of inspection frequency as a function of oxygen and water content

Fig 87 Guidelines for changes in inspection frequency when oxygen or water content is outside preferred

range Source: Ref 224

Any existing tank that has not been so inspected and has been in service longer than 2 years should be inspected at the first opportunity Stress relief after repairs is not recommended As noted previously, it is unlikely to be beneficial and may be harmful

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181 "Columbium," Publication 313-PD1, KBI Division of Cabot Corporation, 1985

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184 M Schussler, Corrosion Data Survey on Tantalum, Fansteel Inc., 1972

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United Kingdom," Chemical Industries Association Ltd., 1980

Corrosion in the Pulp and Paper Industry

Chairman: Andrew Garner, Pulp and Paper Research Institute of Canada

Introduction

IN THE PAST DECADE, the understanding of corrosion phenomena in the pulp and paper industry has progressed from one based largely on practical experience to the point at which an appreciation of the corrosion mechanism is considered

an essential first step to solving a problem Recent cracking problems with kraft continuous digesters provide a good case

in point In the early 1960s, when the continuous digester process was first adopted, the industry had limited familiarity with caustic cracking The occasional report of a cracked batch digester was considered to be an isolated incident, and its significance to the new continuous technology was not appreciated This is evident from the fact that beginning in the mid-1960s, many continuous digesters were built with nonstress-relieved upper cooking zones It is now known that these vessels have caustic levels and temperatures so close to the caustic cracking range that full stress relief is essential Recognition of this was possible only because considerable investigation and research provided a mechanistic understanding of the problem and proposed solutions

A parallel progression of events can be traced for paper machine corrosion phenomena Process changes over the past three decades, including system closure and new brightening practices, have created a series of problems resulting in the failure of existing materials of construction Components with high failure rates include bronze fourdrinier wires, CA-15 cast stainless steel suction press rolls, cast iron vacuum pump impellers, and type 304 stainless steel process piping For most of these components, there has been a sustained effort to understand the failure mechanism, and in some of the later cases, a detailed understanding has been achieved

For example it is now understood that pitting of type 304 stainless steel occurs when the pit site is sufficiently acidified by anions such as chloride or even sulfate; that pitting can be accelerated by thiosulfate, which is thought to deliver sulfur to the pit; and that pitting an be inhibited by bisulfite, which buffers the pit pH The relative proportions of these anions determine whether or not pitting occurs Therefore, the corrosiveness of white water can be predicted and controlled by process changes The current understanding of these and many other corrosion problems in the pulp and paper industry are covered in this article

Paper Machine Corrosion

Donald A Wensley, MacMillan Bloedel Research

Corrosion problems in paper machines are generally most severe in the wet end and in ancillary equipment handling white water (Fig 1) Metal surfaces in the wet end are exposed to the white water environment by immersion, splashing, vapor mist, and the formation of crevices beneath pulp pads or other deposits White water environments cause rapid corrosion of bare carbon steel, appreciable corrosion of copper alloys and cast irons, and can even cause localized corrosion of stainless steels Most wetted paper machine surfaces are constructed from austenitic stainless steels; types 304L or 316L are the predominant choices The critical components in a paper machine wet end are discussed below

Fig 1 Schematic diagram of the wet end of a fourdrinier paper machine

Paper Machine Components

Stock Piping. Paper machine stock piping is typically thin-gage (schedule 10) type 304L or 316L austenitic stainless steel Corrosion is generally not a problem in stock lines if the correct stainless grades and weld filler metals have been chosen for the white water environment and if surfaces are kept clean Surfaces that have become roughened or have weld projections that have not been removed provide sites for stock hang-up Pitting or microbiological attack is likely to occur under deposits Fatigue cracking problems in stock piping are associated with poor welding practice

Headboxes. The headbox (Fig 2) has the critical function of transferring flow formerly contained in a round pipe to a flat flow of uniform consistence across the width of the paper machine Clean, smooth surfaces inside the headbox are essential Aside from the corrosion aspect, the release of pulp pads resulting from stock hang-up may cause product quality problems or even sheet breakage Pickling, passivation, and buffing with abrasives are commonly used to provide

a smooth surface finish Electropolishing improves the finish for surface of 0.4 m (15 in.) and above

Fig 2 View inside a paper machine headbox showing rectifier rolls and overhead shower nozzles Material of

construction is type 304L stainless steel

It is especially important to implement proper cleaning techniques during fabrication Stainless steels are naturally passive

in most papermaking environments; however, the presence of embedded iron, iron oxides, heat tint, weld spatter, slag, and other surface contaminants impairs the corrosion resistance of the underlying metal The misuse of carbon steel wire brushes for weld cleanup may cause serious pitting problems because iron particles will become embedded in the stainless steel surface Iron contamination is best removed by passivating with a nitric acid solution, which dissolves the iron and leaves the stainless steel intact Pickling solutions, which typically contain fluoride in addition to nitric acid, are much more aggressive Pickling solutions also attack the stainless steel in addition to removing iron and weld heat tint; therefore, pickling should be done only as a final cleanup step on equipment for which excellent surface finish is not required

Paper machine headboxes are often constructed from highly corrosion-resistant alloys as additional insurance against the possibility of corrosion roughening of internal surfaces The existence of crevices inside the headbox, the presence of an air-liquid interface, the potential for internal mechanical damage (from rectifier rolls), and even possibility of stray current damage make the headbox environment more severe than that inside stock piping A headbox can be constructed from type 317L stainless steel if type 316L stainless steel is used for other components Overmatched fillers (fillers with higher alloying contents, particularly molybdenum contents, than base metal) have been used to provide additional corrosion resistance for weldments Some headboxes have even been constructed of UNS N10276, a nickel-base alloy that is usually used in much more severe corrosion applications The reasoning behind this material selection is that immunity to corrosion means freedom from maintenance

Headbox apron and slice lips are very prone to pitting corrosion attack (Fig 3) Pits initiate on surfaces outside of the liquid flow and then tend to grow in size, eventually reaching the edge of the lip Corrosion extending to lip edges has a detrimental effect on sheet formation The use of specialty stainless steels, such as UNS S31254 (254SMO), which contains 6% Mo, has been successful in combatting pitting of apron and slice lips

Fig 3 Severe pitting corrosion of a type 316L stainless steel slice lip

Paper Machine Wire. The wire is so named because a phosphor bronze wire mesh was originally used as the moving screen onto which the dilute suspension was distributed from the headbox Bronze wire life was limited by corrosion attack Corrosion inhibitor additions were found to be beneficial Wires coated with tin, nickel, or plastic also gave superior service Synthetic plastic mesh fabrics are now commonly use for paper machine wires Although this eliminates the wire corrosion problem, the use of plastic wires appears to have contributed to the increased corrosion of bronze couch rolls This is attributed to the removal of an effective source of cathodic protection (that is, the metal wires anodically sacrificed themselves and protected the rolls)

Wet End Structures. White water draining through the wire is caught in stainless steel collecting trays and pans Corrosion is likely to occur wherever pulp deposits can accumulate on metal surfaces Structural members are generally protected with stainless steel cladding Where this cladding is incomplete or has been removed, white water can gain access to the underlying structural steel The white water pits or chests beneath the wet end of a paper machine can be of concrete or tile-lined construction

Vacuum Pumps. Liquid ring vacuum pumps typically have ductile cast iron rotors (Fig 4) and gray cast iron casings From a corrosion standpoint, it is desirable to use fresh seal water Paper machine white water is considered unsuitable for seal water use because of its high temperature and low pH Even freshwater may be corrosive if the flow rate is so low that the temperature increases inside the pump or if the freshwater becomes contaminated by excessive white water carryover A discharge seal water temperature of 50 °C (122 °F) and/or a pH less than 4.5 can cause accelerated cast iron corrosion High discharge seal water conductivity (in excess of 200 mho) may indicate excessive white water carryover Accelerated impeller corrosion can occur at rotor tip speeds above 27 m/s (80 ft/s) The presence of sand or grit in the water will also shorten liquid ring vacuum pump life because of erosion

Fig 4 Corroded ductile iron rotor of a liquid ring vacuum pump

As cast irons corrode in seal water or white water a residue of graphite is left behind (Fig 5) Pearlitic ductile iron forms a more protective graphic surface layer than ferritic ductile iron because the three-dimensional carbonaceous residue left behind from pearlite dissolution acts as a binder for the graphite (Fig 6) Rapid liquid ring vacuum pump corrosion can

be expected if environmental, velocity, or erosion conditions are such that the protective graphite film is removed In cases of severe corrosion, liquid ring vacuum pumps have been built with stainless-lined bodies and stainless steel rotors

Fig 5 Cross section of a corroded gray cast iron pipe plug removed from white water service clearly showing

the graphite residue left behind as the iron matrix dissolved

Fig 6 Micrograph of a cross section of a pearlitic ductile iron coupon exposed for 1872 h in vacuum pump

discharge seal water

Suction Rolls. The corrosion-related failures of suction roll shells represent the most serious materials and corrosion problem in modern paper machines A variety of alloys are used for suction roll shells, including bronzes and various grades of stainless steel (martensitic, austenitic, duplex, precipitation hardening) Failure are due to corrosion thinning, pitting, corrosion fatigue, and stress-corrosion cracking (SCC) More detailed information can be found in the section

"Suction Roll Corrosion" in this article

Roll Journals. Paper machine roll journals are subject to corrosion fatigue cracking (Fig 7) Cracking develops at changes in section size most often where journals meet roll heads Depending on the location of the roll, the environment causing corrosion fatigue may be the humid paper machine atmosphere or may involve splashing of white water and/or the formation of damp pads of pulp on the journals Felt roll journals may also be exposed to felt cleaning chemicals, some of which are acidic Journals are constructed from a variety of materials, including medium-or high-carbon steels, low-alloy steels such as AISI 4140 and AISI 4340, cast irons, and various grades of stainless steel No material is immune

to corrosion fatigue; indeed, there is no fatigue limit below which indefinitely long operation could be guaranteed

Fig 7 View of the fracture surface of a felt roll journal that failed by corrosion fatigue Material of construction

is AISI 4140 low-alloy steel quenched and tempered to 300 HB

Improved corrosion fatigue performance can be realized by protecting journals from the environment, by redesigning to increase their diameter and to remove step changes in section size, by polishing to remove surface flaws, and by selecting

a material with superior corrosion fatigue resistance Wet end roll heads and journals often have stainless steel shields to protect them from exposure to white water Coatings, including paint and thermally applied metal spray, have also been used to protect journals The disadvantage of such physical barriers is that they may inspection of surfaces for cracking Duplex stainless steels and precipitation-hardening steels (such as 17-4PH, condition H1100) appear to offer much better corrosion fatigue resistance than the more conventional journal materials Integral roll heads and journals made from ductile cast iron also appear to offer superior service if there is a generous radius between the head and journal

White Water

The wet end of a paper machine produces a uniform paper web from an aqueous suspension of pulp fibers Stock comes

to the paper machine at about 3% consistency and is further diluted to about 0.5 to 1.0% at the fan pump before the headbox Most of the white water is removed by drainage through the paper machine wire The white waters from different sources along the wet end are kept separate and the rich white waters (containing more fibers) are recycled for incoming stock dilution Lean white waters can be used elsewhere on the machine as shower water or as makeup water

Closure of papermaking systems involves the recycle of white water, either directly or after treatment The degree of closure can be expressed as the amount of water consumed per air-dried ton (adt) of paper produced Closed mills may discharge less than 1000 gal/adt per day An open mill, on the other hand, may discharge over 10,000 gal/adt Closure results in an increase in the concentration of dissolved inorganic and organic solids, a decrease in pH, and an increase in temperature

Closure is considered detrimental from a corrosion standpoint; however, there is no clear relationship between increasing closure and paper machine corrosion Although accelerated corrosion may be expected for construction materials that

undergo general corrosion attack (carbon steels, cast irons, copper alloys), no effect on stainless steel corrosion may be noticed until a critical concentration of aggressive ions and/or temperature is attained Increasing closure may only serve

to decrease the margin of safety for corrosion-free service with a given stainless steel Beyond a certain degree of closure, spontaneous pitting of the less corrosion-resistant stainless steels (types 304 and 321) may occur Greater degrees of closure can be tolerated in those mills in which white waters were not particularly corrosive to begin with because the corrosivity of a given white water ultimately depends on its composition

Composition. Paper machine white waters can vary widely in composition from mill to mill, within a given mill, or even from day to day on a given paper machine The corrosivity of white water depends primarily on the pH, temperature, and the concentrations of aggressive inorganic anions, such as chloride and thiosulfate The concentrations of dissolved inorganic compounds found in white waters will depend on whether the wood was seaborne, the type of pulping process used, whether bleaching or brightening was carried out, and the types of chemicals used in the wet end of the machine The chemicals used include such additives as fillers, sizes, retention aids, defoamers, slimicides, and dyes Mechanical pulps are often brightened with sodium hydrosulfite, a chemical that decomposes rapidly in the pulp stream to form other sulfur-containing ions that persist in the white water An average white water composition typical of a West Coast newsprint mill using a furnish of semibleached kraft and hydrosulfite-brightened thermomechanical pulp and groundwood is:

is likely the case for general corrosion of carbon steel, cast iron, and copper alloys; however, to initiate pitting corrosion

of stainless steels, it is necessary to exceed a certain critical temperature This critical temperature will be lower at higher chloride concentrations Closure will contribute to an increase in white water temperature; however, increased temperatures may offer considerable energy savings High temperatures also help to improve drainage in the wet end of a paper machine, thus permitting an increase in machine speed

pH. Although there are both acid and alkaline papermaking processes, most papermaking uses the acid process Typical white water pHs are in the range 4 to 6 The pH is controlled on the acid side by alum additions and/or souring with sulfur dioxide, sulfite, or sulfuric acid

Below about pH 4, the general corrosion rates of many alloys are accelerated Hydrogen ions become so plentiful that their reduction replaces the reduction of dissolved oxygen molecules as the predominant cathodic half reaction in the corrosion process Corrosion processes are no longer diffusion rate limited Decreased pH also lowers the stability of passive films on stainless steels; this makes the steel more susceptible to pitting initiation and increases the probability that a pit, once initiated, will continue to grow

The pH of a white water may decrease upon standing; pH may decrease by 0.5 to 2.0 units within approximately 1 day of sampling The reasons for this decrease are not clear, but it is known that the continued oxidation of dissolved hydrosulfite and bisulfite generates free hydrogen ions One would expect that white waters kept in storage (for example, during shutdowns) may become more corrosive It has been observed that the instantaneous corrosion rate of carbon steel (a measure of white water corrosivity) often correlates well with the magnitude of the pH decrease subsequently measured

in white water samples The white waters showing greater pH decreases are those that originally gave higher carbon steel corrosion rates

Chlorides. Coastal mills that rely on the transportation of logs in seawater may have chloride concentrations ten times higher than those found in inland mills Chloride ions are very stable in white waters and will tend to build up in

concentration as closure proceeds Chlorides can cause pitting and crevice corrosion of austenitic stainless steels Chlorides are also agents for the SCC of stainless steels, but usually at temperatures above 60 °C (140 °F) Stress-corrosion cracking is therefore not a serious problem in most white waters Chlorides accelerate the general corrosion of carbon steels; however, they do not appear to produce significant acceleration of the corrosion of copper-base alloys

Sulfates. Sulfate ions ( ) appear in white water as a residual chemical from the kraft pulping process, from seawater, from alum or sulfuric acid additions, and from the oxidation of more reduced sulfur-containing ions, such as sulfite ( or ), thiosulfate (S2 ), and hydrosulfite (S2 ) Sulfate is the thermodynamically stable form of sulfur in white waters

Sulfate is not an aggressive anion; indeed, it can act as an inhibitor for chloride-related pitting if it is present in (molar) excess over chloride ions On the other hand, sulfate ions make a significant contribution to white water conductivity and therefore indirectly tend to increase the rates of electrochemical corrosion reactions by facilitating charge transfer in the white water electrolyte Sulfate ions also provide the food supply for sulfate-reducing bacteria, which in turn cause microbiological corrosion of paper machine equipment Further, in white waters containing S2 , an excess of

ion is required for thiosulfate pitting to become established

Thiosulfates. Anaerobic decomposition of warm hydrosulfite solutions produces both thiosulfate and bisulfite ions Additions of hydrosulfite in excess of that required to meet target paper brightness levels serve only to introduce even more thiosulfate into the white water system Indeed, it appears that the excess hydrosulfite is very rapidly, and stoichiometrically, converted to thiosulfate The kinetics of oxidation to sulfate are sufficiently slow that appreciable thiosulfate concentrations can build up

Thiosulfate concentrations in white water can be reduced by ensuring that the concentration in the original source is as low as possible Storage of hydrosulfite solutions at 2 to 9 °C (36 to 48 °F) and/or the addition of an alkaline stabilizer to maintain a pH of 10 or more will minimize thiosulfate formation Thiosulfate ions have been found to be particularly aggressive pitting agents for stainless steels (see the discussion "Thiosulfate Pitting" in this section) Pitting can occur at much lower concentrations (for example, 10 ppm Na2S2O3) than that required for chloride pitting (>200 ppm NaCl) Thiosulfates also increase general corrosion rates of copper-base alloys

Sulfites. Sulfite additions are often made to control the pH of pulp stock Bisulfite (the stable form of sulfite in white water) is also produced during the decomposition of hydrosulfite brightening solutions Sulfite is not considered to be aggressive; in fact, sulfites are often used as oxygen-scavenging corrosion inhibitors in other aqueous systems, such as boiler feedwater Sulfites are readily oxidized to sulfate in the presence of dissolved oxygen Sulfites may have inhibitive properties in paper machine white water On the other hand, SO2 evolution can occur under conditions of high sulfite concentration and low pH, leading to severe atmospheric corrosion problems

Conductivity. White water conductivity depends on the concentrations of all dissolved ionic species, both inorganic and organic For carbon steel, cast iron, and copper-base alloys, higher conductivity is an indication of higher corrosivity This

is not the case for stainless steels as long as they are passive and are not undergoing localized corrosion attack

The corrosivity of carbon steel due to paper machine white water has been monitored by taking instantaneous linear polarization corrosion rate measurements together with simultaneous white water samples for analysis Linear regression

of the carbon steel corrosion rate with individual white water properties reveals that conductivity provides the best

regression coefficient, r2:

• Chloride pitting and crevice corrosion

• Thiosulfate pitting

• Microbiological attack

In recent years, there have been reports of rapid attack of type 304 stainless steel equipment in mills where this alloy had served well for decades The recent unsuitability of type 304 stainless steel can be attributed to closure, hydrosulfite brightening, or both

Chloride Pitting and Crevice Corrosion. Stainless steels rely on the stable formation of a passive surface film for immunity to corrosion in paper machine white waters Oxidizing conditions must prevail for the passive film to form and

be maintained Dissolved oxygen from the air is sufficient to maintain stable passivity Stainless steels can also withstand slightly reducing conditions in white waters without suffering serious attack

There is a certain safe range of oxidizing conditions within which the stainless steel corrosion potential can vary (these oxidizing conditions can be measured electrochemically as a range of oxidizing potentials) In the presence of aggressive anions such as chlorides, however, this safe potential range is narrowed If a certain critical corrosion potential (called the breakdown potential) is exceeded, chloride ions can attack the stainless steel surface The attack manifests itself as pits because passive film breakdown occurs only in isolated locations, such as weak spots in the film due to defects in the underlying metal Pitting attack is favored by the following conditions:

• Higher chloride concentration

• Higher temperature

• Highly oxidizing conditions

• Low pH

• Stagnant or low-velocity conditions

• Lower molybdenum content in the stainless steel

A new chloride pit tends to repassivate unless the favorable conditions for initiation are maintained until the pit has become established The metal within the pit becomes anodic with respect to the surrounding metal outside, which becomes cathodic because of ready access to dissolved oxygen for the reduction half of the net corrosion reaction Anodic dissolution and subsequent hydrolysis of metal ions inside the pit result in the generation of free hydrogen ions, which in turn promote the diffusion of additional anions into the pit to maintain charge neutrality Because chloride anions are much more mobile than sulfate anions, chlorides preferentially migrate into the pit Eventually, the solution inside a growing pit may come to resemble a solution of hydrochloric acid more than paper machine white water, and pit growth may continue almost independently of external conditions

Crevice corrosion initiates beneath deposits or in other areas shielded from direct contact with the white water environment (Fig 8) Crevice corrosion initiates more readily than pitting attack, which it closely resembles, because the

conditions for an oxygen concentration cell already exist The metal surface inside a crevice has difficulty maintaining passivity because of reduced access to dissolved oxygen; on the other hand, the surrounding metals has ready access to dissolved oxygen Passive film breakdown within the crevice is thus facilitated by the natural tendency of the crevice to become anodic and the surrounding metal to become cathodic Once crevice corrosion is initiated, its growth mechanisms

is the same as that for pitting corrosion

Fig 8 Crevice corrosion of a type 316L checking plate located adjacent to a headbox apron Corrosion

developed under pulp pads that formed despite the highly polished surface

The primary corrosion concern with white water system closure is that the critical chloride concentration and/or temperature for stainless steel breakdown will be exceeded Under such conditions, pitting corrosion may occur spontaneously This is a particular concern with type 304L stainless steel Stainless steels are chosen for service in chloride pitting environments on the basis of molybdenum content Because higher molybdenum alloys are also more expensive, it is common to select the alloy with the minimum molybdenum content required for resistance to pitting A hierarchy of austenitic alloys, representing increasing resistance to chloride pitting and crevice corrosion, can be listed in increasing order or minimum molybdenum content:

Above the range represented by the ratio, there is insufficient thiosulfate to reach the pit nucleus Below this range, there

is too much thiosulfate reduction, which prevents acidification of the pit

Thiosulfate corrosion particularly affects those grades of stainless steels that do not contain molybdenum Once formed, pits are very stable and are not subject to spontaneous repassivation Scratches encourage the initiation of pits Few, large pits tend to form rather than many, small pits, as in chloride pitting Sensitized type 304 stainless steel (weld heat-affected zones, HAZs) is particularly susceptible to thiosulfate pitting Type 316L stainless steel is the minimum grade that should

be used for white water service where high thiosulfate levels may exist It is recommended that thiosulfate levels be controlled below 5 and 10 ppm for equipment made of type 304 and 316 stainless steel, respectively

Microbiological Corrosion. Paper machine white waters contain nutrients that can sustain bacterial growth Microbiological growth thrives in near-neutral pH environments White water temperatures are also usually within the favorable range of 40 to 50 °C (105 to 120 °F) Although higher temperatures may prevent the growth of some forms of bacteria, increased temperatures can increase the metabolism of those bacteria that can adapt to heat The result is the formation of slimes

Stock and white water flow systems are designed to minimize slime accumulations Surfaces are polished and weld projections removed to prevent hang-ups Wherever slime deposits can build up, however, microbiological corrosion can occur Once a deposit has grown to a sufficient thickness to exclude oxygen, a colony of sulfate-reducing bacteria

(Desulfovibrio desulfuricans) can become established Enzymes produced by these anaerobic bacteria catalyze the

reduction of sulfates to form free sulfide ions Chemically reducing conditions quickly develop, resulting in the depassivation of the stainless steel surface beneath the deposit Active corrosion in the form of pitting then proceeds

Corrosion due to Desulfovibrio desulfuricans is manisfested by the presence of large, shallow pits covered with a black

crust (Fig 9) Perforations through stainless steel equipment are usually small because the entry of oxygen at a leak will stop the activity of sulfate-reducing bacteria (Fig 10)

Fig 9 Section of type 304L stainless steel plate removed from a tapered header used to deliver stock to a

headbox showing severe microbiological corrosion

Fig 10 External view of the type 304L stainless steel tapered header in Fig 9, showing leakage occurring at

small perforations

Corrosion Control in Pulp Bleach Plants

Andrew Garner, Pulp and Paper Research Institute of Canada

Pulp mill bleach plants have traditionally used austenitic stainless steels because of their combination of good corrosion resistance and weldability Type 317L (18Cr-14Ni-3.5Mo) has been the typical bleach plant alloy for oxidizing acid chloride environments However, bleach plants have become more corrosive over the past 20 years as mills have closed wash water systems and reduced effluent volumes In modern closed bleach plants, type 317L is no longer adequate for long-term service (Ref 1), and many mills have turned to higher-alloy stainless steels, nickel-base alloys, and titanium for better corrosion resistance Metals are chosen over nonmetals for moving equipment, such as washers Metals are stronger, tougher, have better fatigue properties, and, if they have sufficient corrosion resistance, require virtually no maintenance However, the more corrosion-resistant alloys are more costly, and the challenge is to choose an alloy with just enough resistance to avoid corrosion problems

A wide selection of alloys is available for bleach plant applications The list includes three families of stainless steels (austenitic, ferritic, and duplex), whose differing merits are outlined in Table 1 Table 2 list most of the commercially available candidate alloys and their chemical compositions Table 3 outlines the influence of each alloy component on bleach plant corrosion resistance

Table 1 Characteristics of three families of stainless steels for bleach plant service

Family Examples Characteristics Comments

316L 317L 904L 254SMO AL-6XN

Tough, ductile, readily welded without loss of corrosion resistance; corrosion resistance related to alloy content

Bleach plant steels traditionally chosen from this group

Higher alloys have remarkable corrosion resistance Thin-section (<3 mm, or 0.120 in.) may find applications as corrugated deck or tubing; not common at present (C + N) 0.025%

29-4C NYBY MONIT

Ti- or Ti +

Nb stabilized

type

CURE

SEA-0.02% C max versions Less expensive 0.02% C version of

low-interstitial ferritics, for thin-section weldments ( 1.14 mm, or 0.045 in.)

Table 2 Typical chemical analyses of commercially available bleach plant alloys

VDM Cronifer 19/25 HMO 21 25 1.3 0.02 0.14 0.3 0.018 0.010 5.9 bal

Ferritic stainless steels

Hastelloy alloy G 22 bal 1.3 0.01 0.4 0.014 0.002 6.4 19.8 (q)

Hastelloy alloy G-3 22 bal 0.7 0.01 0.3 0.011 0.002 7.1 19.9 (r)

Inconel 625 22 bal 0.1 0.01 0.2 0.010 0.007 9.5 3.9 (s)

Hastelloy alloy C-276 16 bal 0.5 0.01 0.1 0.011 0.002 15.6 5.6 (t)

Hastelloy alloy C-22 22 bal 0.5 0.01 0.1 0.011 0.002 13.0 3.0 (u)

Beneficial alloy additions

Chromium Enhances resistance to initiation of pitting and crevice

Higher levels of nickel enable partly corroded component

to remain functional; little or no nickel in ferritic grades

Molybdenum Enhances resistance to initiation and propagation of pitting

and crevice corrosion

Over three times more effective than chromium against pitting and crevice attack, but has solubility limit of about 7% in stainless steels

Nitrogen Enhances pitting resistance, particularly in combination

with molybdenum

Used in austenitic and duplex grades only; increases strength of the steel

Detrimental residual elements

Carbon More than 0.03% can cause sensitization, making

heat-affected zones of welds less corrosion resistant

Oxidized out of steel during refining, down to limit set by simultaneous, costly, chromium oxidation

Phosphorus Can cause hot cracking, that is, cracks formed in weld

metal upon cooling Hot cracks are sites for crevice corrosion, which looks like pitting attack

Can only be controlled by use of low-phosphorus charge materials Less than 0.015% P is respectable

Sulfur As with phosphorus, can cause hot cracking Can be controlled to very low levels (<0.005% S) by good

steel-making practice Less than 0.015% S is respectable Less than 0.005% S is excellent

Note: Silicon, manganese, and copper are added for steelmaking reasons or sulfuric acid resistance (copper)

Austenitic stainless steels, including the AISI 300 series and enriched variations of these steels, are tough and easy to weld Their corrosion resistance ranges from fair to excellent, depending on the alloy content (Table 1)

Ferritic stainless steels in the 400 series are not used in bleach plants, because of their poor corrosion resistance, particularly after welding However, there is a new generation of extralow carbon and nitrogen (low interstitial) grades that retain postweld corrosion resistance As with austenitic stainless steels, the corrosion resistance of these ferritic stainless steels ranges from fair to excellent, depending on alloy content However, steels such as 29-4 have not been used

in the bleach plant, because of problems with embrittling precipitation in thicker sections (3 mm, or 0.12 in., and over) and because of the special precautions required to avoid nitrogen contamination during welding

In contrast to the ferritic grades, the properties of austenitic and duplex stainless steels are enhanced by nitrogen As a result, duplex stainless steels such as 2205 and Ferralium 255 have recently been developed with improved corrosion resistance

Laboratory Assessment of Candidate Alloys

The relative corrosion resistance of stainless steels and nickel-base alloys can be assessed with the ferric chloride (FeCl3) test, which was recently shown to be an appropriate assessment of bleach plant performance (Ref 1) The relative resistance to pitting corrosion of a range of commercial stainless steels is shown in Fig 11 A critical temperature has been measured for each alloy below which no pitting will occur in FeCl3 (Ref 2, 3) The higher the cited pitting temperature, the more resistant the steel is to pitting Steels such as type 316L and 317L, for example, have comparatively poor pitting resistance The 904L-type alloys with about 4.5% Mo provide somewhat better pitting resistance, and the 6%

Mo steels, such as 254SMO, are remarkably resistant Based on these results, one might predict that the duplex steel Ferralium 255, and the manganese-substituted austenitic, Nitronic 50, should outperform type 317L in the bleach plant

Fig 11 Effect of molybdenum content on the FeCl3 critical pitting temperature of commercial stainless steels The more resistant steels have higher critical pitting temperatures

Generally, similar conclusions can be drawn from crevice corrosion tests in FeCl3 (Ref 4, 5) Such data are presented in Fig 12 and 13 The critical temperatures for crevice corrosion are lower than those for pitting, indicating that crevice corrosion is more readily initiated If equipment is not designed to avoid crevice corrosion, then this will be the mode of failure An example of this form of attack is shown in Fig 14, which shows a type 317L corrugated deck from a C-stage washer that failed because of crevice corrosion

Fig 12 Effect of molybdenum content on the crevice corrosion temperature of commercial stainless steels The

more resistant steels have higher crevice corrosion temperatures in the FeCl3 test

Fig 13 Effect of molybdenum content on the crevice corrosion temperature of nickel-base alloys Note the

superior performance of Inconel alloy 625 and Hastelloy alloy C-276 Compare with Fig 12

Fig 14 Failure of corrugated type 317L washer-deck commonly associated with crevice corrosion

Field Testing of Candidate Alloys

Field testing of candidate alloys has been performed by a number of researchers (Ref 1, 6, 7, 8, 9) An early study measured pitting attack on a range of welded alloys in comparatively benign Nordic bleach plants (Ref 6) Pitting and crevice attack were later tested in more closed (more corrosive) Canadian bleach plants This program focused on a few representative alloys, examined the selection of welding electrodes, and compared gaseous and liquid exposure in C- and D-stage washers (Ref 1) Figure 15 shows the corrosion products covering stainless steel test coupons that were exposed

to a D-stage washer environment

Fig 15 Stainless steel coupons of type 316L, 317L, and 904L on a rack exposed below the incoming stock of a

D-stage washer Profuse ferrous oxide corrosion products cover the coupons

Two other exposure programs of note were carried out by the Technical Association of the Pulp and Paper Industry (TAPPI) Corrosion and Materials Engineering Committee in U.S and Canadian mills Unwelded alloys were tested in the first program (Ref 7), while the second program tested welded alloys (Ref 8, 9) The alloys tested included almost all available choices for bleach plant application However, alloy development has been such an active field in recent years, other promising steel, such as Ferralium 255, AL-6XN, and 29-4C, have been commercialized since the comprehensive TAPPI exposure programs

Data from all these test rack programs can be interpreted as follows:

• The premium bleach plant alloy of the future: It will probably be chosen from 254SMO, Hastelloy alloy G-3, Sanicro 28, or 20Mo-6; VDM Cronifer 19/25 HMO, AL-6XN, and 29-4C (thin section) were not tested, but they should also be competitive

• Attractive alloys close to the type 317L cost level: Nitronic 50 and 1.4439 (317LMN) are promising

alternatives to type 317L Ferralium 255 was not tested but should also be competitive

• Alloys with less competitive price of performance: Titanium, Hastelloy alloy C-276, Inconel alloy 625, and 29-4-2 all performed exceptionally well, but are expensive; 904L and related alloys appear to perform slightly below the level required for a premium alloy

Bleach Plant Environments

Residual oxidants, such as chlorine (Cl2) and chlorine dioxide (ClO2), are the primary cause of corrosion in the bleach plant The corrosive influence of Cl2 has been demonstrated with coupon testing at higher Cl2 levels (Fig 16) (Ref 10); other work has indirectly identified 25 ppm Cl2 or ClO2 as the level above which corrosion reactions are driven by residual oxidants (Ref 11) It seems probable that the 25 ppm of Cl2 determined by iodine titration is close to zero actual

Cl2, because the titration is also sensitive to traces of oxidizing organics present in C-stage filtrates

Fig 16 Effect of residual chlorine on the corrosion rate of test coupons in the C-stage washer

Any steps that can be taken to lower residual oxidants to below 25 ppm will lower corrosion rates Options include automatic chlorine control; chlorine dioxide sensor/controls, SO2 antichlor, and NaOH additions (ClO2 is not very corrosive at pH 7, and NaOH additions to a pH 4 filtrate will transform ClO2 to after a few hours)

Recycling of filtrates can compound corrosion problems For example, residual ClO2 can be recycled with a D2-stage filtrate to the D1-stage washer showers; therefore, an increase from 50 to 150 ppm ClO2 has been measured in a D1-stage filtrate when D2-stage SO2 additions were cut off during high ClO2 usage Recycling of D1- or D2-stage filtrate to the C-stage washer should be avoided completely, because the more acidic C-stage filtrate (pH 2) regenerates ClO2 from chlorite ions, thus rendering the shower water highly corrosive to stainless steels Many C-stage washers were lost because of this practice when recycling was first carried out

Effect of Temperature, Chlorides, and pH

Temperatures and chloride ion concentrations are raised after bleach plant closure (Ref 1) The influence of these two changes on corrosion, although significant, is usually far overshadowed by the corrosive effect of residual oxidants Typical environmental conditions for bleach plant washers are given in Table 4

Table 4 Typical environmental conditions for bleach plant washers

Environment Chlorination Chlorine

Recycling can lower pH in all acidic washing stages Lower pH will create a corrosion problem only during the last stage

of bleach plant closure, namely when C-stage filtrate is used for C-stage tower dilution With this practice, lower pH can give the pulp the viscosity protection required for the consequent higher-temperature chlorination However, when pH decreases to 1.5 or 1.2 as compared to the normal pH 2, stainless steel corrosion rates increase dramatically It is probable that a critical lower limit pH exists for any given stainless below which corrosion rates are high, but such limits have not yet been identified with any degree of certainty What can be assumed is that more highly alloyed materials should have lower limiting pHs and that nickel and molybdenum should be the most influential alloying elements

Vapor-Phase Corrosion

The corrosion of nonwetted components, such as shower pipes and metal vats above the stock level, is a major problem in C-stage washers This attack is caused by excess chlorination, in which gaseous chlorine from the filtrate makes small droplets of condensation highly corrosive Chlorination stage gas-phase attack is probably the most aggressive in all the bleach plant Methods of avoiding this problem include improving chlorine control; using titanium or nonmetallic-coated shower pipes; and, for stainless steel vats, cladding with 6% Mo stainless or nickel-base alloys to just below the liquid level or lining the whole vat with nonmetallic coating (good design and workmanship is essential)

The vapor space above D-stage washers is not very corrosive unless SO2 is used in excess, is poorly controlled, or is badly mixed When both SO2 and ClO2 are present in the vapor space, any condensation will contain hydrochloric acid (HCl) and sulfuric acid (H2SO4), mixtures of which are very corrosive to stainless steel

D-Stage Washer Corrosion

Sodium hydroxide or SO2 additions are made to bleached pulp before the pulp machine in order to improve drainage and

to limit brightness reversion Such additions are often made before the D2 washer so that the washer may also be protected from residual-related corrosion In some cases, SO2 additions are made before the D1-stage washer for corrosion protection, and in Nordic countries, SO2 is even added before some C-stage washers Sulfur dioxide additions are an effective (if costly) way of limiting corrosion; they work because SO2 reacts irreversibly with the residual oxidant

to form nonoxidizing reaction products, for example, with ClO2 (Ref 1):

2ClO2 + 5SO2 + 6H2O 2HCl + 5H2SO4

The amount of acid formed by this reaction has a negligible effect on washer corrosion, and sufficient SO2 should be used

to maintain a trace of residual SO2 at all times Sulfur dioxide control can be automatic or manual; in the latter case, a target maximum pH of 3 is often used for additions before the D2-stage washer

If SO2 additions are discontinued for example, because of SO2 shortages rust nodules or "barnacles" may appear within

a few weeks on the washer (Fig 15) Residual ClO2 (present because SO2 was discontinued) has caused the iron oxide [FeO(OH)] corrosion deposits to form (Ref 1) The deposits are insoluble above pH 3.5, and their presence exacerbates the corrosion problem, because additional under-deposit formation occurs Deposits can be avoided by increasing the pH

to above 5.5 (target pH 7) with NaOH and by holding residual ClO2 to less than 25 ppm

When a D-stage washer drum has been in operation for a number of years it is not advisable to change to C-stage service

if there is any evidence of rust deposit The deposits may plug corrosion pits, and in C-stage washing at pH 2, the deposits will be dissolved quickly, causing the washer to leak

Hypochlorite, Oxygen, and Peroxide

Because bleaching with these three oxidants is usually carried out under alkaline conditions, stainless steels are much less subject to pitting the crevice corrosion Hypochlorite washers are commonly made of type 316L to resist crevice attack Oxygen reactors are often made of high-nickel stainless steels such as 20Cb3 to guard against chloride SCC, which can occur at higher temperatures in pressure vessels Some lower alloys have been used in this latter application, apparently without problems, although type 304 stainless steel has failed by chloride SCC However, chloride pitting corrosion can also occur in oxygen reactors, and alloys containing higher molybdenum levels may be necessary Alkaline peroxide is used to brighten mechanical pulps in newsprint production without any corrosion consequences Peroxide has also been used in place of ClO2 in chemical-pulp bleach plants and appears to present no more corrosion problems than ClO2

In ferrite-free stainless steel weld metal, for example, fillers commonly recommended for 904L(a)

Use IN112 electrode for 3 to 6%

Mo austenitics

Lack of penetration In one-side or stitched butt-weld joints Ensure full penetration, and do

not use stitchwelds on process side

In niobium- or titanium-stabilized steels A very rare problem Niobium-

and titanium-stabilized steels not common

(a) Particularly after high heat input welding

Welding without filler metal creates a preferential attack site on austenitic stainless steel and should be avoided in washer construction It is important not to select a filler metal that gives a deposit that is less corrosion resistant than the base metal For type 316L, the American Welding Society standard filler is adequate (Ref 13) However, for type 317L and the more highly alloyed materials, recent field and laboratory tests have shown that a filler metal with a composition similar

to that of the base metal can have much lower pitting resistance (Ref 1, 14)

The optimal weld metal for all 4.5 to 6% Mo austenitic stainless steels is Inconel alloy 112 or an equivalent (for example, Inconel alloy 625 or Avesta alloy P12) Inconel alloy 112 is a good choice because:

• It is metallurgically compatible with all austenitic stainless steels

• As-welded Inconel alloy 112 is highly resistant to pitting and crevice corrosion

• There is no significant galvanic effect between Inconel alloy 112 and austenitic stainless steels in bleach plant liquors

• If microfissures or hot cracks occur in the weld metal, they will not be preferentially attacked by crevice corrosion This is a particular problem for most ferrite-free stainless steel fillers

Microfissuring or hot cracking is a phenomenon associated with thermal stresses during welding These stresses usually cause small cracks to form in the weld metal or HAZ of a stainless steel or nickel-base alloy weldment Higher nickel content alloys, which have a greater coefficient of thermal expansion, are more susceptible to hot cracking Cracking is more likely to occur because of higher phosphorus (>0.015% P) and sulfur (>0.015% S) in the alloy or contamination of the weld area It is most commonly seen in the HAZ in the previous pass of a multiple-pass weld Hot cracking rarely has

a detrimental effect on the mechanical properties or structural integrity of a fabrication However, it can be very detrimental to the corrosion properties of a weldment Microfissures form crevice corrosion sites that are readily attacked

Recent laboratory tests have shown that other fillers can be used for 904L, such as Sanicro 27.31.4L CuR, Smitweld NiCro 31/27, and Thermanit 30/40 E These fillers would be appropriate for microfissure-free shop construction However, contaminated weldments are best repaired with Inconel alloy 112 or an equivalent filler

Another common problem with stainless steel weldments sensitization in the HAZ is avoided in bleach plants by the use of low-carbon steels (0.03% C max for austenitics) Similarly, fusion-line attack (sometimes called knife-line attack) due to precipitation of carbides at the fusion line in niobium- and titanium-stabilized austenitic steels is rarely seen, because these steels have been made obsolescent by new steelmaking technology

However, attack at the fusion line is possible when overalloyed fillers such as Inconel alloy 112 are used with high heat input welding Such welding can create zones consisting of melted base metal that is not mixed with weld filler called unmixed zones at the fusion line Cases of unmixed zone corrosion have occasionally been observed in the bleach plant

In practice, this can be minimized by the use of lower heat input on the final weld passes

Electrochemical Protection

The discussion thus far has centered on the selection of material or the control of the environment to minimize corrosion

A third approach has recently become available with the development of electrochemical protection for bleach plant washers (Ref 10, 15, 16)

General Description. The life of a stainless steel washer can be greatly extended if the washer is cathodically polarized from the oxidizing potentials imposed by residual oxidants such as chlorine (Fig 17) to a more negative, passive potential by the use of a rectifier and a platinized anode mounted in the washer vat (Fig 18) Electrical contact to the washer is made through a rotating mercury contactor; the washer potential is measured with a reference electrode and

is automatically controlled with a feedback-controlled rectifier to a potential set point or window that minimizes corrosive attack

Fig 17 Effect of residual Cl2 concentration in the washer vat on the free-corrosion potential of a type 317L stainless steel bleach plant washer Source: Ref 15

Fig 18 Components of a washer electrochemical protection system

Principle of Protection. A detailed description of the principle of electrochemical protection is provided in Ref 15 However, the essential feature of the technique is the use of the electron as an antichlor for corrosion protection Electrons are fed from the rectifier to the washer in the form of electric current At the washer surface, they react with chlorine (Ref 2):

Cl2 + 2e- 2Cl

-Therefore, like SO2, electrons react with the corrosive oxidant (chlorine) to form relatively harmless chloride ions For corrosion protection, the electron has a clear advantage over SO2: It can be delivered to the cathodic reaction site For this reason, comparatively few electrons are needed The required current is low, and running costs are negligible

Because of the comparatively low cost of electrochemical protection and the ease of retrofitting to existing washers, commercialization of the technique has found wide acceptance The first successful operation of one of these systems was

in Nova Scotia, Canada, in 1978 By the end of 1986, there were about 90 installations worldwide

Monitoring Technique. The corrosion rate of each protected washer is monitored with coupons, using a technique designed for comparison of protected and unprotected coupons (Ref 17) A mounting bolt is welded to the end face of the rotating washer as shown in Fig 18, and two coupons, together with segmented crevicing disks, are mounted so that one

is in electrical contact with the washer and the other is isolated, with all other mounting details being identical (Fig 19)

The degree of protection is assessed by comparing the corrosion of protected and unprotected coupons after a 60-day exposure period (Fig 20)

Fig 19 Crevice corrosion monitor assembly

Fig 20 Type 317L stainless steel monitor coupons after a 60-day exposure attached to a C-stage washer The

coupon on the left was in electrical contact with the washer and was therefore protected The coupon on the right was isolated and unprotected Substantial crevice corrosion occurred on the right-hand coupon at 7 of the

20 possible crevicing sites

Results to Date. Both weight loss and depth of attack measurements have been made on coupons to assess the comparative severity of pitting or crevice corrosion (Ref 10) The ratio of unprotected and protected coupon weight loss is used as a measure of protection Table 6 lists protection ratios measured for washers that have been protected for up to 5 years (6 washers types/212 test coupons) These results show that protection lowers corrosion rates On the average, unprotected coupons lose six times more weight than protected coupons

Table 6 Type 317L test coupon data from the first six electrochemical protection systems

Washer Protection

period,

years

Average protection ratio(a)

Improved performance with protection can be compared with improved performance after alloy upgrading in the absence

of protection The results of an extensive bleach plant exposure program, which involved 880 coupons, 40 test racks, 10 mills, and no electrochemical protection (Ref 1), were reexamined to obtain the average weight loss ratios (using type 317L as a base case) for alloys 904L and 254SMO These data are given in Table 7; greater ratios indicate better performance Type 317L coupons lose 5.5 times as much weight as 254SMO coupons Therefore, the improved performance achieved by the electrochemical protection of type 317L appears to be very close to that gained by upgrading type 317L to unprotected 254SMO

Table 7 Test rack weight loss data for unprotected steels

Steel Average weight loss ratio (a)

Type 317L 1

904L 2.5

254SMO 5.5

(a) This ratio is obtained, for example, by determining the

average weight loss for type 317L coupons ÷ average

weight loss for 904L coupons (larger ratios indicate

better performance)

Economics. The installation and operating costs of electrochemical protection systems are small compared to the resulting cost savings Capital cost savings are such that if the life of a washer is extended from 5 to 10 years by protection, the protection system will have a payback period of about 1 year

Even this substantial saving can be overshadowed by savings in chemical costs in mills that add NaOH or SO2 to pulp before washing Experience has shown that, in general, SO2 additions need not be made ahead of a protected washer to protect it from corrosion If pulp souring is still required, then this can be done immediately after the washer, where much less SO2 will be needed Additional savings can be realized by eliminating SO2 use in a closed bleach plant, because SO2free recycled filtrate used for tower dilution consumes much less unreacted ClO2 Protection systems have been installed

on washers made from types 316L, 317L, and 904L, and 254SMO stainless steels A protected 254SMO washer probably represents the state-of-the-art for corrosion control in the most severe washer environments

Corrosion by Sulfite Pulping Liquors

C.B Thompson and Andrew Garner, Pulp and Paper Research Institute of Canada

Sulfite pulping, one of the oldest methods of chemical pulping, dates back to the 1860s For many years, sulfite pulping was the primary chemical method for making paper, although since the 1950s it has been overtaken in importance by the

kraft process to such an extent that the sulfite industry was perceived by many as dying out In recent years, however, a growing appreciation of the versatility of the sulfite process and the appearance of combined sulfite and mechanical pulping methods (semichemical or chemi-mechanical) have combined to ensure the future of sulfite pulping These changes in the industry are thoroughly documented in Ref 18)

This section will discuss the various methods of sulfite pulping, the principal corrosion mechanisms occurring in sulfite environments, and the major corrosion problem areas found in sulfite mills Information on corrosion in the disk refiners used in the later stages of semichemical and chemi-mechanical pulping can be found in the section "Corrosion of Mechanical Pulping Equipment" in this article

Sulfite Pulping

Sulfite Chemistry. Sulfite pulping liquors are prepared by dissolving SO2 in a solution of calcium, sodium, magnesium, or ammonium hydroxide The pH of the resultant cooking liquor depends on the base and the amount of SO2dissolved, and it can range from 1 to above 13 The exact value is chosen according to the particular mill process and the desired pulp characteristics Fresh sulfite liquor is essentially a mixture of sulfite and bisulfite ions in an aqueous solution

of SO2 The ratios of these three components can vary widely, according to the liquor pH The relative concentrations of bisulfite and sulfite ions a different pH levels are shown in Fig 21; bisulfite dominates at pH values less than 6, while sulfite ions dominate at pH values above 7 The concentration of aqueous SO2 also increases at lower pHs (Ref 19)

Fig 21 Relative concentrations of bisulfite ( ), sulfite ( ), and aqueous sulfur dioxide (SO 2 ) as a function of liquor pH at 130 °C (265 °F) Aqueous sulfur dioxide and bisulfite dominate at acidic pHs, sulfite dominates at alkaline pHs Source: Ref 19

Traditional Sulfite Pulping. For many years, most sulfite mills used a calcium hydroxide base cooking liquor, mainly because of the economy of the calcium carbonate feedstock The calcium-base process is restricted to very low pH (typically 1.5) and low cooking temperatures (140 to 150 °C, or 295 to 300 °F) because of scaling and solubility problems Furthermore, the process is difficult to adapt to high-yield pulping These reasons, as well as increasingly stringent antipollution legislation, have led to the expanded use of soluble sodium, magnesium, and ammonium bases in recent years

A typical sulfite operation is shown in Fig 22 Sulfur (either in powder or liquid form) is burned to produce SO2 The gas

is cooled rapidly to minimize the formation of sulfur trioxide (SO) and then passed into absorption towers, where it is

dissolved in the hydroxide base to form the raw pulping liquor Pulping is carried out in either batch or continuous digesters; when the cook is complete the chips are released into a blow tank The chips are then taken for washing, screening, and bleaching as required The spent cooking liquor (red liquor) can be evaporated and burned in a furnace for steam generation and/or recovery of cooking chemicals, or it can be further treated to obtain chemical by-products A typical operation might also include equipment for SO2 recovery from the cook and an acid accumulator

Fig 22 Typical layout for a traditional low-yield sulfite mill operation

Recent developments in sulfite mills have centered around the need to decrease pollution, to increase pulping efficiency (that is, increase the proportion of usable fiber obtained per cook), and to adapt to market demands for specific grades of pulp These requirements have resulted in a variety of processes, such as high-yield sulfite pulping and chemi-thermomechanical pulping, in which sulfite chemical treatment of the chips is combined with additional treatment of the fiber in a disk refiner The interrelationship of these process with pure chemical pulping and pure mechanical pulping is shown in Fig 23 An important practical point to note is that high-yield and very high-yield sulfite pulping require the use

of a digester, but chemi-mechanical and chemi-thermomechanical pulping use smaller impregnation vessels or steaming tubes for pretreatment of the chips

Fig 23 Interrelationship between chemical, semichemical, and mechanical pulping processes The processes

marked with an asterisk use impregnation vessels or steaming tubes rather than digesters Source: Ref 18

Corrosion Mechanisms in Sulfite Liquors

Stainless steels are used for most process equipment in the sulfite mill As in any industrial plant, the steels in sulfite mills can experience many different types of corrosion The most serious ones are usually connected in some way to the presence or absence of SO2 Chloride-induced localized corrosion and SCC may also be problems, particularly in coastal mills

The presence of SO2 in sulfite liquors can affect corrosion of stainless steels in three ways It can:

• Maintain passivity against acidic cooking liquors

• Form H2SO4 by decomposition of the liquor

• Form H2SO4 by oxidation to SO3

These effects are discussed in more detail below

Maintaining Passivity. Sulfur dioxide in solution helps to maintain the passivity of stainless steels such as types 316L and 317L that are commonly used in sulfite mills (Ref 20, 21) Therefore, a potential corrosion problem exists in any situation in which the concentration of dissolved SO2 becomes very low This may happen, for example, in vacuum evaporators or in batch digesters during a cook However, practical experience has shown that this type of corrosion is usually not of concern with batch digesters Unless conditions are especially severe (that is, high temperature, low pH, high chlorides), austenitic stainless steels containing more than 2.7% Mo can withstand short periods without any dissolved SO2 present Opening the digesters to the atmosphere between cooks also helps to maintain passivity

Sulfuric Acid Formation by Liquor Decomposition. Acid bisulfite liquors can spontaneously decompose to form

H2SO4 (Ref 22):

3SO2 + 2H2O 2H2SO4 + S 3NaHSO3 NaHSO4 + Na2SO4 + S + H2O