Failure Analysis Case Studies II Episode 10 pot

If in the present case of a two-liquid water-butanone mixture, the partition of sodium chloride between the water-rich phase and the butanone-rich phase is of a similar magnitude, calcul

The 'lower' and 'upper' layers in Table 2 refer to the water- and acetone-rich solutions, respec-

tively Comparison of the data in Table 2 and the water-butanone phase diagram shown in Fig 1

shows that the solubility of acetone in water is roughly comparable to that of butanone in water, and that water is more soluble in acetone than in butanone Thus, the solubility of sodium chloride

in water-acetone mixtures should only be considered as a guide to what might happen in water- butanone mixtures Although the lower and upper layers in the immiscible water-acetone mixtures are in equilibrium, and the activity of the sodium chloride in each layer is therefore equal, inspection

of Table 2 shows that in acetone-water mixtures the partition coefficient for sodium chloride

between the water-rich lower layer and acetone-rich upper layer is 40: 1

If in the present case of a two-liquid water-butanone mixture, the partition of sodium chloride between the water-rich phase and the butanone-rich phase is of a similar magnitude, calculations indicate that the concentration of sodium chloride in the water-rich phase will be higher than in both the butanone-rich phase and the original 8 wt% solution of distilled water in butanone

Table 2 Solubility of sodium chloride in aqueous solutions of acetone at 20 "C

Lower layer Upper layer

304

'l'he weight of sodium chloride in the 168 kg of the liquor used in the batch process is 0.084 kg With a partition coefficient of 40:1, this amount of sodium chloride would be partitioned thus: the water-rich layer would contain 0.082 kg, and the butanone-rich layer would contain 0.002 kg The calculations given previously show that addition of the minimum volume of water required to cause separation of the 8 % water-in-butanone solution used in the batch process into two immiscible liquids results in the formation of 0.23 1 of the water-rich phase Using the data shown in Table 2

as a guide, the greatest amount of sodium chloride which could dissolve in 0.231 of water-rich solution is 0.045 kg Thus, if only 0.23 1 of the water-rich phase had been formed, it is probable that sodium chloride would have precipitated from the water-rich phase

Since no solids had been observed when the system was dismantled, it appears that at least 0.5 kg

of water-rich phase must have been formed This led to a reappraisal of the amount of water which had been introduced to the system Calculations based on the Lever rule indicated that this would have required the addition of just under 4 kg of water to the feed liquor

Although the strict applicability of these calculations to the present case may be questioned, since data for the solubility of sodium chloride in water-acetone mixtures have been used, they do suggest that the hypothesis is tenable The result would have been that the pump which had suffered the crevice corrosion was not exposed to a very dilute solution of chloride but to a brine, and crevice corrosion of 3 16L would inevitably have occurred

Fig 3 Light ring of rust which appeared at the O-ring immersed in the water-rich phase after 12 days of

305 Table 3 Chemical analysis of the stainless steel rod used in the demonstration: composition in wt%

UNS S31603 0.03 2.00 1 .oo 0.045 0.030 16.0-18.0 10.0-14.0 2.00-3.00

maximum maximum maximum maximum maximum

observed in either the butanone-rich phase or the original 8 wt% water-in-butanone mixture These observations show that crevice corrosion had just begun to initiate in the water-rich phase, but had not initiated in either the 8wt% solution of water in butanone, or in the butanone-rich phase formed after the addition of water

The hypothesis presented in the discussion is tenable, and can explain how crevice corrosion of 316L occurred in what was supposed to be a very dilute solution of sodium chloride in an 8 wt% solution of water in butanone

The unresolved question was how the additional water required to cause separation of this solution into two immiscible liquids was introduced into the system

REFERENCES

I Francis, A W., LiquicCLiquid Equilibriums Interscience, New York, 1963

2 Seidell, A,, Solubilities oforganic Compounds, 3rd edn Van Nostrand, Princeton, NJ, 1941

3 Seidell, A., Solubilities oflnorganic Compounds, 3rd edn Van Nostrand, Princeton, NJ, 1941

Failure Analysis Case Studies II

D.R.H Jones (Editor)

0 2001 Elsevier Science Ltd All rights reserved 307

TYPE I PITTING OF COPPER TUBES FROM A WATER

DISTRIBUTION SYSTEM

PAUL0 J L FERNANDES

Advanced Engineering and Testing Services, MATTEK, CSIR, Private Bag X28 Auckland Park, 2006,

South Africa (Received 9 Augusf 1997)

Abstract-Samples of copper tubes from a cold water distribution system which had failed due to pitting whilst in service were subjected to a detailed failure investigation Analysis of the tubes showed that failure was a result of Type I pitting attack While the exact cause of pitting was unknown, it was hypothesised that

it could have been due to changes in the water quality and/or content The tubes were found to be made from

phosphorus de-oxidised copper and no anomalies were evident in either the chemical composition or the

microstructure which could have caused the pitting observed It was recommended that the tubes be replaced and that due attention be given to ensure that the new tubes are free of internal carbonaceous deposits or other foreign matter 0 1998 Elsevier Science Ltd All rights reserved

Keywords Corrosion, pitting corrosion

1 INTRODUCTION Copper tubes are used extensively in water distribution systems due to their corrosion resistance and ease of installation In Europe and North America they account for more than 80% of all tubes installed in water service [l], amounting to over 100 million metres of tubing In spite of these large quantities, tube failures are relatively rare Of the failures that do occur, pitting corrosion accounts for approximately 60%

This study presents an investigation of the failure of copper tubes from a cold water distribution system carrying potable water in a shopping centre The tubes, which were built into the brick walls, sprang leaks in several premises in the shopping centre after approximately 12 years’ seMce, causing severe staining of the walls Examination of the tubes revealed the presence of pin holes perforating the tube walls

2 EXPERIMENTAL PROCEDURE

2.1 Visual examination

Several tubes sections were received for analysis These were sectioned to reveal the internal surfaces, which were found to be covered with a greenish-white scale (Fig 1) Furthermore, localized deposits of green corrosion product in the form of tubercules were also evident (see arrow in Fig I) Some tubercules were carefully removed by light scrubbing to reveal the underlying metal A shiny, black layer of an unidentified compound was found to exist beneath the greenish-white scale Beneath this black layer, in turn, pits penetrating into the tube wall were found An example of the various layers and the underlying corrosion pit is shown in Fig 2 Some of the pits observed were relatively large and deep, as shown in Fig 3

2.2 Chemical analysis of internal scale and corrosion products

Samples of the tubes were examined in a scanning electron microscope (SEM) equipped with an energy dispersive spectroscopy of X-rays (EDS) facility The results of the EDS analysis of the greenish-white scale found on the internal surfaces of the tubes are shown in Fig 4 The large copper Reprinted from Engineering Failure AnaZysis 5 (l), 35-40 (1 998)

80E

309

Fig 3 The internal surface of a tube showing extensive pitting ( x 3)

Fig 4 The EDS results of the greenish-white deposits found on the internal surfaces of the tubes

3 METALLOGRAPHY Samples from the tubes examined were prepared for metallographic analysis using standard grinding and polishing techniques Etching was carried out in acidified ferric chloride The typical microstructure observed in all cases consisted of large equi-axed grains, indicating that the tubes were in the annealed condition

310

3.1 Chemical anaZysis

An analysis of the chemical composition of the tubes was carried out using a wet chemical analysis method From the high phosphorus content it was evident that the tubes were made from phosphorus de-oxidised copper

4 TYPE 1 PITTING Pitting corrosion is the most common failure mechanism for copper tubes in water distribution systems Essentially two different types of pitting attack have been identified, and these are referred

to in the literature as Type I and Type I1 pitting* The former is known as cold water pitting and occurs more frequently than the latter

Type I pitting is usually encountered in cold water systems carrying borehole or well waters free from organic matter [I] It occurs sporadically and can result in tube wall penetration within a few months In some cases, however, penetration occurs only after 15 years or more The internal surfaces of tubes undergoing Type I pitting are usually covered with a greenish scale of a copper compound called malachite Beneath this scale, the tube surface is covered with a smooth, shiny layer of dark cuprite which is very friable and easily spalled off Pits are usually associated with the presence of tubercules which form over pin hole defects in the cuprite layer

The characteristics of Type 1 pitting attack are such that many pits at all stages of development can usually be found [I] Larger pits are generally linearly arranged along the bottom half of horizontal water lines When pits are very close together, tubercules can extend over a number of pits to form one long tubercule Although pitting has been observed in annealed, half-hard and hard-drawn tube, susceptibility is generally greatest in the annealed condition The pits formed are usually saucer-shaped and relatively wide

A number of causes of Type I pitting have been identified [I] Firstly, the incidence of pitting has been associated with the presence of carbonaceous films on the internal surface of the tube These films are residues of the lubricant used for the drawing operation and which are carbonized during annealing The quantity and distribution of these films on the internal surface appears to affect the severity of pitting The problems arising from the presence of these carbonaceous films can be overcome in practice by scouring the tubes with a water-sand or a water-air blast

Secondly, pitting has been associated with the presence of foreign matter deposits on the bottom half of horizontal tubes [l] This is in agreement with observations on the preferential location of pits discussed above The foreign matter deposits can be introduced into the water lines in a number of ways Metal chips and filings and dirt can be allowed to contaminate the system during installation If these are not properly removed before service, they may deposit along sections of the water lines where the water velocity is low Foreign matter deposits may also be introduced into the system in the water or may be due to corrosion products formed during surface corrosion of the tubes during service The concentration of these deposits, and hence their deleterious effects, can be reduced by the installation of filters in the water line

Thirdly, another factor said to cause pitting attack is the presence of soldering pastes on the insides of the tubes This generally results from bad workmanship and can be avoided by ensuring that adequate quality standards are maintained during installation The soldering pastes may act as deposits in the same way as foreign matter Alternatively, during soldering or brazing these pastes may be converted to oxides which form as a thin film on the copper surface These oxides are gcncrally cathodic to copper and can therefore give rise to pitting corrosion

The effect of water quality on the incidence of Type I pitting is the subject of some controversy and no consensus has been reached in this regard Some general observations have been made, however, on the effects of various constituents and characteristics of water on the extent of pitting,

*Some researchers have also reported the existence of Type 111 and Type IV pitting, but these appear to be variations of

4 N O Is the ratio of sodium t o nitrate greater than I ?

Table 1 The effect of various water constituents and characteristics on Type I pitting

Carbon dioxide (CO,)

Assists pit initiation and growth, but its effect depends on the concentration of other chemical species

Essential for pitting attack Assists the breakdown of protective surface films and results in the formation of wide, shallow pits

Increases in pH generally decreasing the probability of pitting

Increased 0, content increases the probability of pitting

Increased CO, content increases the probability of pitting due to a decrease in pH

and these are summarised in Table 1 An empirical screening process has also been developed to assess the risk of Type I pitting in various waters [2] (Fig 5) This process has been used extensively with reasonable success

A number of characteristics of Type I pitting discussed above were evident in the failed copper

tubes from the shopping centre The presence of tubercules of corrosion product and the greenish scale on the internal surface of the tubes were clearly evident (Fig 1) The friable underlying layer

of shiny, dark cuprite was also observed (Fig 2) The wide, saucer-shaped pits and their approxi- mately linear distribution were also evident and are shown in Fig 3 It is also evident that pits at various stages of development were observed

It was concluded that the failure of the copper tubes was due to Type I pitting attack It is not

clear at this stage what the exact cause of pitting failure was, particularly given the fact that pitting

only became evident after 12 years’ service It is highly unlikely that it may be due to the presence

of foreign matter deposits introduced during installation of the system The introduction of foreign matter in the water is, however, a possibility, particularly if the water is not filtered A change in water quality or content (e.g resulting from mixing of the water with borehole or well waters) could also be responsible for pitting

Once initiated, pitting attack can in some cases be halted through the application of appropriate treatments of the water and the metal The extent of pitting observed in the present case, however,

0

z

312

suggested that such treatment would be both unsuccessful and unfeasible It was therefore rec- ommended that the copper tubes be replaced Careful attention should be given to the usual causes

of Type I pitting In particular, it should be ensured that all tubes be thoroughly cleaned and freed

of any carbonaceous deposits prior to installation The tubes should also be cleaned to ensure complete removal of any foreign matter deposits and solder pastes after installation The use of water filters could also be considered to prevent the introduction of foreign matter in the water Furthermore, the quality and content of the water should be determined and its potential to cause pitting assessed The extent of replacement or modifications to the water distribution system would,

to some degree, depend on the results of such water analyses

REFERENCES

1 Internnl Corrosion of Water Distribution Sysrem Report of Cooperation Research, AWWA Research Foundation, USA,

2 Billiau, M., Drapier, C., Muteriaux et Techniques, Nos 1 and 2

1985

Failure Analysis Case Studies II

D.R.H Jones (Editor)

0 200 1 Elsevier Science Ltd All rights reserved 313

CORROSION OF FLEXIBLE WAVEGUIDES

D PAPATHEODOROU, M SMITH and 0 S ES-SAID*

Mechanical Engineering Department, Loyola Marymount University, 7900 Loyola Blvd, Los Angeles,

CA 90045-8145, U.S.A

(Received 9 August 1997)

Abstract-Waveguides are commonly used in spacecraft subsystems to convey signals After noticing a transponder ouput power drop, borescope inspection of a flexible waveguide revealed a green contaminating residue on silver plated brass and copper sections Analysis revealed that the residue, primarily copper hydroxy nitrate, Cu(OH),N03, was created by exposure of the plating to nitric acid Possible sources of nitric acid include inadequate cleanliness after parts were exposed to a nitric acid containing silver bright dip, or high temperature electrical arcing in the presence of air and moisture Whatever its source, it is suggested that the waveguide be plated with a more corrosion resistant metal such as rhodium 0 1998 Elsevier Science Ltd All rights reserved

Keywords: Corrosion, electronic-device failures, surface coatings

1 INVESTIGATION Flexible waveguides, common in spacecraft payload sub-systems, transport signals between various units (e.g., filters, transponders, and converters) During preliminary testing at ambient temperature and pressure, an output power drop was detected within a signal generating unit of a waveguide system Green contamination residue was found in the waveguides An investigation commenced

to characterize the corrosion and determine its cause

The flexible waveguide, Fig 1 , has a rectangular thin wall cross section having corrugations which allow it to be formed The green residue was found on brass and copper surfaces, primarily in the bottom of those corrugations (dark bands in Fig 2) In some areas, the waveguide wall had corroded through

To determine the material damage severity, as well as the composition of the residue, an analysis

of samples taken from the waveguide was conducted using visual inspection, optical microscopy, scanning electron microscopy and X-ray methods Samples were prepared by cutting and spreading open the waveguide to expose its internal surfaces containing many voids and much debris (Fig 3)

Fig 1 Profile of waveguide Dark bands are low points in waveguide or corrugations

*Author to whom correspondence should be addressed

Reprinted from Engineering Failure Analysis 5 (l), 49-52 (1998)

Fig 2 Inner surface of waveguide

L_cx ,

Fig 3 Debris on waveguide surface Copper hydroxy nitrate corrosion along corrugations in waveguide

A chemical analysis using X-ray diffraction analysis, subsequently verified by Fourier transform infrared spectroscopy and energy dispersive X-ray spectroscopy, revealed that the debris is primarily copper hydroxy nitrate Cu(OH),N03 To determine how it got there, a laboratory test was per- formed to try to create the same debris on clean waveguide samples by placing on them a small amount of nitric acid Two hours later, a blue color was observed in the acid After about 12 h, blue crystals began forming at the silver plated interface After 5 days, most of the solution had been replaced by green corrosion analyzed as a copper hydroxy nitrate Nitric acid clearly caused the corrosion Its source could be either faulty fabrication processes or arcing induced chemical reactions

2 FABRICATION PROCESS The silver plating on the brass waveguide is applied after the brass has undergone a multi step surface preparation process First, the brass surface is cleaned and etched in a caustic cleaning solution for 5-60 s After subsequent rinsing under running tap water, the brass is immersed in a bright dip solution for 5-20 s This removes scratches and oxide, making the brass look shiny The bright dip solution is composed of 5-10% tap water, 60-75% sulfuric acid, 20-35% nitric acid To

remove the bright dip, parts are washed in running tap water The use of pumice and a brush is required for assemblies The bright dip vendor specifies that this cleaning technique is suffcient Once bright dipping is complete, parts should be first immersed in clean running water, then boiling hot water, and then dried To avoid contamination between one dip operation and another, parts should be rinsed in running water, hot water and then dried at each step [l]

Both silver and copper bright dips exist to make either copper or silver shiny Once silver plating was complete, a bright dip step may have been inadvertently included despite its lack in vendor process specifications For instance, a silver bright dip may have been performed to relieve the effects of poor silver plating, inadvertently leaving behind an acidic residue

3 ARCING INDUCED CHEMICAL REACTIONS

If nitric acid was indeed produced by arcing, nitric oxide (NO) would need to be present Colorless and noncombustible, nitric oxide can be produced from atmospheric oxygen and nitrogen in the presence of an electric arc In this instance, such production is possible-there was evidence of arcing on the waveguide surface In addition, arcing could have initiated pits in the silver plating, exposing the underlying copper bearing base material to chemical attack

A similar incident of corrosion in an aircraft waveguide system occurred about 20 years ago In that case, arcs were created in a clean noncorroded waveguide while gas samples were taken for an analysis by mass spectrometry An analysis of two samples is shown in Table 1

Table 1

Mole percent Sample #I

28 pm 18.708 1.022 0.443

316

This confirms that nitric oxide could be formed by electrical arcing Despite the small concen- tration, it still exceeds that normally found in air by several orders of magnitude Nevertheless, even

if such nitric oxide is present, it must react with moisture for nitric acid to form With humidity

controlled between 30 and 6O%, available evidence suggests that the current waveguide system was not exposed to excessively moist conditions

4 RECOMMENDATIONS Silver is attacked by nitric acid and will be corroded by reducing acids in the presence of oxidizing agents [2] Nitric acid is a strong oxidizing agent It oxidizes all metals except gold, platinum, rhodium and iridium [3] In strong acid solutions, the hydrogen is continuously evolving as bubbles from the corroding metal and this process continues until either all the metal or acid is consumed

If the waveguide system is operated in conditions in which moist air is not absorbed during operation and if the system is purged of any nitric oxide after operation, the corrosion can be eliminated It would also be best to pass the air through a dryer prior to introducing it into the waveguide to guarantee moisture levels are minimized

For improved protection against nitric acid formation, if arcing does occur, it is best to electroplate with noble metals other than silver, since it is attacked by nitric acid The noble metals have extremely high corrosion stability and do not rely on the formation of an oxide coating Their high cost and low strength limits their use to thin films and liners on other structural materials [4] They are economical for numerous corrosion applications Platinum is resistant to nitric acid at all temperatures and concentrations [ 5 ]

Electrodeposited platinum is reasonably dense and generally adheres well Mechanical and physi- cal properties depend greatly on plating conditions and thin coatings are used for corrosion and

wear resistant electrical contacts [5] Gold is very good in dilute nitric acid and strong sulfuric acid

Rhodium electroplates well and is used for critical valve parts and other applications where total

resistance to an aggressive environment is necessary [2] A 37% rhodium 63% nickel alloy has

better resistance to general corrosion than 14 carat yellow gold [5] Rhodium finds most of its applications as an element in platinum to which it imparts added corrosion resistance to many

acids In general, electrodeposition has been employed for thin rhodium coatings A rhodium

thickness of 5 x 1OP6-2O x loP6 inch over silver minimizes tarnishing [5] In this case, these metals can be used for protection against nitric acid if formed due to arcing

Quality control during waveguide manufacturing must guarantee that no nitrate ions are on the waveguide In bright dipping, a small amount of metal is corroded but the part has a shiny finish

as opposed to a dull oxide coating Speed of operation and uniformity are the essentials of bright dipping The acid acts very quickly and long exposure time will result in more corrosion After dipping the parts should be very quickly rinsed in cold water and then hot water and dried [I] Pure, clean water, e.g distilled water, is undoubtedly the best for making solutions It is very difficult for small amounts of silver nitrate to dissolve in water that has impurities in it However,

in distilled water the silver nitrate will perfectly dissolve to a clear solution [I] Water taken from

wells is sometimes found unfit for the best results in plating, if it contains lime or is strongly

mineralized with iron, sulfur or magnesium

Acknowledgement-The authors are grateful to Ms Rachel Adams of the Mechanical Engineering Department of Loyola

Marymount University for typing the paper

REFERENCES

1 Hawkins, H J., The Polishing and Plating of Merals, Lindsay Publication, Bradley, IL, 1987, pp 98-100

2 National Association of Corrosion Engineers, (N.A.C.E.) Corrosion Basicx, An Introduction N.A.C.E Publication

3 Waser, J., Trueblood, K N and Knobler C M., Chem One, McGraw Hill, New York, 1976, pp 80

4 Butler, G and Ison, H C K., Corrosion and irs Preuenlion in Waters, Reinhold Publishing Corp., New York, 1966, pp

5 Metals Handbook Committee Metals Handbook, Vol 1 8th edn, American Society For Metals, Metals Park, OH, 1961, Texas, 1984, pp 590

101

Failure Anaiysis Case Studies 11

degradation

J.M Henshaw".", V Wood", A.C Hallb

The University of Tulsa, Department of Mechanical Engineering, 600 South College Avenue, Tulsa, O K 74104, U.S.A

The University of IIIinois, Materials Science and Engineering, Urbana, IL 61801, U.S.A

Received 30 July 1998; accepted 9 September 1998

of three distinct failure mechanisms can result: (1) the belt fails to latch, (2) the belt will latch but will not unlatch, and (3) the belt appears to be latched but is not The seat belt mechanism, and the ways in which the degraded button can cause it to fail, are described in detail The buttons themselves were found to have been injection molded of ABS and to have undergone photo-oxidative degradation This degradation process

is documented and described Conclusions from the analysis and lessons learned from the failures are

described, along with the auto industry's short- and long-term solutions to the problem 0 1998 Elsevier Science Ltd AH rights reserved

Keywords: Photo-oxidative degradation; Polymer degradation; ABS; Seat belts

1 Background

the largest formal automobile recall in the history of the industry While news reports in the

vehicles were from Honda and Nissan Toyota alone among the U.S and Japanese carmakers was

* Corresponding author

Reprinted from Engineering Failure Analysis 6 (l), 13-25 (1 999)

318

not affected.) The cost to carmakers should a mandatory recall become a reality was estimated at Ultimately, this incident did not result in a formal mandatory recall by the U.S National Highway Traffic Safety Administration The manufacturer of these seat belts, the Takata Corpor- ation, and the affected automakers agreed to a voluntary recall of these vehicles The following

The Reason for This Notice: Honda has determined that front seat belt buckle release buttons

seatbelts made by the Takata Corporation These seat belt buckle release buttons are made of red plastic, and are marked PRESS If a button breaks, pieces may fall into the buckle assembly

If this occurs, the buckle may not operate properly, thereby creating a safety risk To prevent this problem from occuring, Honda will replace all broken front seat belt buckles, free of charge

In addition Honda will modify all unbroken buckles manufactured by Takata to prevent future button breakage

Under the terms of the voluntary recall, owners of affected vehicles were asked to take their cars

to their dealer who would perform an inspection and then either replace or modify the seat belts The details of the inspection and modification procedure are described later in this report

2 The seat belt mechanism and its failure

2.1 The seat belt mechanism

While there are some variations in design among the various models of Takata-manufactured seat belts affected, the basic mechanisms are quite similar All include a release button, which is part of the seat belt receptacle mechanism, that is adjacent to the slot into which the seat belt clasp fits when the belt is engaged A typical Takata seat belt receptacle of the affected design is shown

In order to understand the function of the release button, and how it contributes to the various system failures, it is necessary to first understand how the seat belt receptacle mechanism works

four key parts of the mechanism are shown and named (Other parts are omitted for simplicity.)

mechanism where it encounters a polymeric ‘slider’ The clasp forces the slider to compress a spring In the bottom part of the figure, the ‘latch’ and ‘locking slider’ rotate counterclockwise into the locked position when the opening in the clasp becomes aligned with the male portion of the latch As the latch rotates into the locked position, the locking slider slides to the left into a position where it is constrained along with the latch (by the unshown housing) from rotating back

to the unlatched position To release the belt, the release button is pushed against the locking slider, sliding it back out of the way of the housing, and allowing both the locking slider and latch

to rotate clockwise until the male portion of the latch no longer engages the opening in the clasp Finally, the compressed spring behind the slider can extend itself, ejecting the clasp from the mechanism