Handbook Corrosion (1992) WW Part 10 ppsx

4 Effect of cobalt content and grain size on the transverse rupture strength of WC-Co cemented carbides Cemented carbides are not selected for corrosion applications per se.. Corrosion-

Fig 1 Microstructures of WC-Co (a, c, and e) and WC-TaC-TiC-Co (b, d, and f) cemented carbides In a, c, and

e, the white areas are cobalt binder phase In b, d, and f, the darker, more rounded grains are the WxTayTizC cubic solid-solution phase (a) and (b) Fine grain structures (c) and (d) Medium grain structures (e) and (f) Coarse grain structures All 1500× Source: Ref 1 and 2

The first key to the successful development of cemented carbides was that these refractory metal compounds, particularly

WC, are best produced as powders In fact, the only logical way to produce tungsten is the hydrogen reduction of WO3 or ammonium paratungstate powder into tungsten metal powder The carburization of tungsten to WC also results in a fine powder The second key was the discovery of the eutectic system WC-Co (Fig 2) Liquid-phase sintering is possible well below the melting point of the WC and even below the melting point of cobalt

Fig 2 Quasi-binary phase diagram for the WC-Co system

Cemented WC is produced by mixing from 3 wt% or less up to as much as 30 wt% of cobalt metal powder with a balance

of WC powder The mixed powders are ball milled, generally in volatile solvents, for times ranging from a few hours to

as long as 7 days Alternatively, the powders are milled in an attritor for 1 to 10 h

A suitable transient binder is added to the powder, which is then pelletized and pressed to form the shape Finally, the part

is sintered at temperatures between 1300 and 1600 °C (2370 and 2910 °F), most often in vacuum Because a liquid phase

is formed during sintering, virtually 100% density is achieved More information on the production of cemented carbides

is available in the articles "Cermets and Cemented Carbides" and "Production Sintering Practices" in Powder Metal

Technologies and Applications, Volume 7 of the ASM Handbook

Effect of Composition on Properties

The two most common variables in cemented carbides are the cobalt or binder content and the grain size As shown in Fig 3, increased grain size decreases hardness, and increased cobalt content also decreases hardness (Ref 6) Increased contents of cobalt or other binders, however, are necessary to increase strength As shown in Fig 4, strength increases with increased cobalt content; although a maximum appears to occur at about 15 to 18% Co, this is true only for transverse rupture strength (Ref 6) Very high impact strength requires very high cobalt contents (up to 25 or 30 wt%) and coarse-grain carbide In corrosion applications, however, the binder content ranges from virtually nil (there are some so-called "binderless" compositions that actually contain 1 to 2% binder) up to about 10%, with exceptions running up to 15% binder

Fig 3 Effect of cobalt content and grain size on the hardness of WC-Co cemented carbides

Fig 4 Effect of cobalt content and grain size on the transverse rupture strength of WC-Co cemented carbides

Cemented carbides are not selected for corrosion applications per se They are extremely important in corrosion

conditions in which high hardness, wear resistance, or abrasion resistance is required When this is the case and the selection of a cemented carbide is logical, the corrosion-resistant properties are examined For ordinary corrosion resistance, many metals and ceramics are better choices, but when wear resistance is also a requirement, the cemented carbide is needed

Binder Composition and Content. The corrosion resistance of cemented carbides is based on the two very different components The cobalt binder has very poor corrosion and oxidation resistance, and the WC has excellent corrosion resistance and good oxidation resistance Alternate binders, such as nickel, have better corrosion resistance than cobalt and are used in spite of their lower hardness and strength Nickel is a superior binder for cemented TiC and therefore is

used in all cemented TiC materials regardless of the need for corrosion resistance In some applications, cemented TiC shows repair corrosion resistance, and in other applications, cemented WC is better

The addition of nickel to the usual cobalt binder used for WC, or the substitution of it entirely for cobalt, always improves corrosion resistance There is, however, a sacrifice in strength, hardness, and wear resistance A chromium addition also enhances corrosion resistance

The most important variable in the corrosion of cemented carbides is the binder content Because the binder corrodes more than the carbide, the smaller the amount of binder the better On the other hand, decreasing the binder decreases the strength

Carbides. Additions of TaC and TiC to the WC-Co materials are common for the compositions used for machining steel These additives give the carbide crater resistance Cratering on the top of a metal-cutting insert is the result of a physicochemical reaction The addition of TaC and/or TiC will slow this reaction; indeed, it has been found that TaC also enhances the outright chemical corrosion resistance of these materials

Other additives, such as chromium carbide (Cr2C3), molybdenum carbide (Mo2C), niobium carbide (NbC), and vanadium carbide (VC), are often added in small quantities as grain growth inhibitors Little has been published about their effect on corrosion, but chromium has been shown to be a beneficial binder additive to WC-Ni binder compositions (Ref 7) Vanadium carbide and Mo2C will probably have a weakening effect on the strength of a WC-base hardmetal

For TiC-base hardmetals, Mo2C is invariably added to the composition, but there are no known studies of the effect of molybdenum on corrosion resistance The molybdenum has always been added to enhance the liquid-phase sintering of the TiC-base compositions In general, these compositions have been made for their hardness and strength characteristics, with corrosion resistance being a secondary consideration Most rescent TiC-base compositions have titanium nitride (TiN) added, and this has been shown to improve the corrosion resistance (Ref 8)

Perhaps it is not surprising that compositions developed primarily for machining should show improved corrosion resistance In machining, there is heat with resultant oxidation and often corrosionlike mechanisms Thus, some of the improved machining compositions also show better corrosion behavior On the other hand, optimum corrosion resistance

is obtained by tailoring the composition and amount of the binder phase This can result in lower-strength materials with limited usefulness in machining applications

Because carbon is the basis of cemented carbides, its variation within a given composition is very important to properties and corrosion resistance Figure 5 shows the range of carbon content allowable in the simple WC-Co compositions as cobalt content is varied (Ref 9, 10, 11) Corrosion-resistant compositions have three problems:

• The lower the cobalt or binder content, the better the resistance to corrosion, but this limits the safe zone, in which neither carbon porosity nor phase (hard, brittle M6C or M12C intermetallics) exist

• The lower the carbon content, the better the corrosion resistance, but falling into the -phase zone results in embrittlement of the material

• The addition of alternate binders, such as nickel, decreases the safe zone

In making corrosion-resistant cemented carbides, manufacturers must be aware of these problems and limitations Information on the metallography and microstructures of these materials is available in the article "Cemented Carbides" in

Metallography and Microstructure, Volume 9 of ASM Handbook, formerly 9th Edition Metals Handbook

Fig 5 Effect of cobalt content and carbon content on the phases present in WC-Co cemented carbides

Applications of Cemented Carbides

The major applications of cemented carbides actually involve environments that are inherently corrosive For example, the major use of cemented carbides is for metal-cutting (machining) applications In these applications, extreme heat is generated whether or not coolants are used, and in those cases in which coolants are used, the corrosive attack of the coolant is a factor in the performance of the cutting tool In general, however, very little heed is paid to this factor; cemented carbides are more often chosen for their wear resistance in such applications as mining and oil well drilling In actuality, there is a corrosive environment to be contended with in mining (Ref 12) and oil well drilling; the natural waters and other fluids involved are often very corrosive Other well-known examples in which cemented carbide is performing

in a corrosive environment include balls for ball point pens and dental drills In both of these examples, the corrosion resistance of the most frequently used WC-6Co composition was serendipitous The material was selected for its wear resistance It just happens to have good corrosion resistance in the saline and ink solutions The dulling of cemented carbide saw tips used for sawing green or unseasoned wood is a corrosive as well as a wear phenomenon (see the section

"Saw Tips and Corrosion" in this article)

Examples of the use of cemented carbide in true corrosion applications include the following:

• Ball point pen balls

• Dental drills and burrs

• Surgical and orthodontic tweezers, pliers, and clamps

• Valve seats

• Valve balls and valve stems

• Valve and shaft seals (seal rings)

• Ball mill linings and balls

• Internal parts in industrial meters

The article "Cermets and Cemented Carbides" in Powder Metal Technologies and Applications, Volume 7 of the ASM

Handbook contains more information on applications for cemented carbides

Selection of Cemented Carbides for Corrosion Applications

The selection of cemented carbides is a very difficult problem for the user There has been a lack of standardization on the part of the producers, and this lack has not been answered by any national or international standards organization Some attempts have been made to standardize with regard to metal-cutting applications There is International Organization for Standardization (ISO) standard 513 for metal-cutting applications for carbide (Ref 13) It is widely used in Europe and most other parts of the industrial world, but it is not recognized in the United States (Ref 11) In addition, there is no ISO standard for cemented carbides used for wear, mining, or corrosion applications, and if any exist in other industrialized countries, the producers choose to ignore them, or they may be so broad that a given producer can have three or more grades falling into one category (Ref 14, 15)

The producers also tend to disregard attempts at standardization in the hope of having a unique product Even in the established WC-6Co grades, the producers offer several different varieties based on different grain size or different minor element additions For example, the company that developed the WC-6Co composition about 70 years ago offers five different grades of this composition, and two of them have identical published properties They are not alone In some cases, three grades are shown with the same composition and properties A good example is the nine differently designated 6% Co compositions of one company Five of the nine are indeed different because of small TaC additions or grain size, but one of the compositions has three designations, and two of them have two designations More often, the reason for the multiple designations of the same composition is that one designation is for cutting tools, another for wear parts or dies, and another for mining Another problem area is the selection of composition by the manufacturer For example, if one producer establishes a WC-25Co composition, another producer will make and market a grade with 24%

Co, and another a product with 26% Co Despite these problems, Table 2 lists the properties of various representative grades for corrosion applications; Table 3 lists approximate compositions and proprietary designations for a number of corrosion-resistant grades

Table 2 Some physical properties of corrosion-resistant cemented carbide grades

Properties of a carbon steel, a tool steel, and a cast cobalt alloy are included for comparison

Abrasion resistance factor(a)

Coefficient

of thermal expansion,

Titan 50 66.5 22.5 1.0 10.0 K602(c) 88.2 1.8 10.0 94.3 15.6 759 110 4.9 K701(c) 85.8 10.1 4.1 92.0 14.0 1138 165 6.5 62.8 0.15(d) K703(c) 93.3 5.8 0.9 91.5 14.7 1931 280 4.5 K714(c) 88.4 6.1 4.5 1.0 92.5 13.1 1827 265 1.8(d) 4.0 K801(c) 93.7 0.3 6.0 90.0 14.8 2103 305 17(d) 5.6 96.2 0.23(d)

Special corrosion resistance

K803(c) 89.0 1.0 10.0 91.0 14.4 2000 290 5.6

Grades for heading and forming dies

HD-15(b) 85.0 15 87.4 14.10 3172 460 30 6.5 83.7 0.20 HD-20(b) 80.0 20 85.3 13.60 3103 450 45 6.8 83.7 0.20

Table 3 Representative compositions and proprietary designations of corrosion-resistant cemented carbide grades oprietary designations

Representative composition, % Proprietary designations

WC Co TaC TiC Ni Cr Mo 2 C Kennametal Krupp

Table 3 includes grades from 16 manufacturers worldwide These are meant to be representative only in a general sense There are well over 100 manufacturers throughout the world (over 25 in the United States alone); therefore, it is not feasible to include all In addition, cross comparisons are not precisely possible For example, a grade listed with an approximate composition of WC-25Co may be cross referenced with a comparable grade that contains only 24% Co Reference 15 contains more complete data on any grade, and manufacturers can be consulted for more information

In addition to small differences in cobalt content from one manufacturer to another, there are small differences in minor additives and in grain size For example, with the 6% Co grades, there are two basic grain size classes fine and coarse but these two are not precisely the same from one manufacturer to another Some have a slightly finer or coarser size within the defined category of fine and coarse Again, precise standards are lacking References 1, 2, and 18 are the attempts at standardization, but they are useful only in a general sense; moreover, no producer ever refers to the specifics

of these standards in designating the cemented carbide it produces Another factor is the intentional addition of minor elements such as tantalum, titanium, vanadium, chromium, and molybdenum as grain growth inhibitors or the inadvertent introduction of minor amounts of these and other elements in the raw materials or through recycling These elements affect hardness and strength and cannot be discounted in the selection of a cemented carbide for corrosion applications

Other processing variables also affect properties and performance Among the important results of processing variables is the amount of porosity in the final cemented carbide product In some cases, the porosity is negligible, and theoretical density is achieved In other cases, porosity is present This can be rated in accordance with ISO 4505 (Ref 19) or ASTM

B 276 (Ref 20), both of which are based on the same standard photomicrographs The ultimate in freedom from porosity

is achieved by hot isostatic pressing This operation, when carried out properly at about 138 MPa (20,000 psi) and at temperatures of 1200 to 1400 °C (2190 to 2550 °F), has no detrimental or beneficial effect on the cemented carbide except for the removal of the last vestiges of porosity Table 4 shows some typical values of selected mechanical properties of cemented carbide grades that may also be helpful in selecting a proper composition for a particular application

Table 4 Selected mechanical properties of corrosion-resistant cemented carbide grades

Charpy V-notch impact resistance(a)

Tensile strength Compressive strength Modulus of elasticity

Corrosion in Aqueous Media

The corrosion of cemented carbides is based on the solubility of the key ingredients used in the various compositions Although some alloying occurs, the solubility of the WC or TiC in cobalt or nickel is very limited The main alloying in the WC-Co compositions is primarily based on the addition of TiC, TaC, and NbC, which form cubic-phase solid solutions with WC

Table 5 shows the relative solubilities of the chief constituents of cemented carbides in various media Tungsten carbide is insoluble in most acids as well as in basic and salt solutions It is soluble only in very strong mixtures of nitric acid plus hydrochloric acid (HNO3 + HCl) and HNO3 plus hydrofluoric acid (HF) Cobalt and nickel show the same significant solubility in all acids Even so, the nickel binder compositions show somewhat less attack in some acid solutions than the cobalt binder alloys From this elementary information, it is obvious that the lower the binder content, the less the corrosion

Table 5 Relative solubilities in acids and bases of the basic constituents of cemented carbides

Medium and solubility(a)

Alkali solutions

Salt solutions

(a) Solubility: V, very soluble; Sl, slightly soluble; I, insoluble; S, soluble

(b) Data from Ref 21 and Ref 22 are contradictory

Corrosion of cemented carbides, therefore, is generally based on the surface depletion of the binder phase such that at the surface region only a carbide skeleton remains; because the applications are invariably for wear or abrasion, this skeleton

is rapidly worn away At low binder phase contents, the rate of attack is diminished, and in conditions in which the corrosion is not too severe, the reduced binder content will be beneficial In more severe corrosion, however, the use of a cobalt binder is prohibited, and the WC-Co grade is simply not resistant enough In these cases, certain corrosion-resistant grades should be used

The most common of these are WC with nickel alloy binders and TiC-Ni-Mo2C-base cemented carbide Figure 6 shows the corrosion rate as a function of pH for these different types of cemented carbides tested in buffered solutions These tests included a final surface wear treatment by tumbling in order to obtain a true value of the depth of the corroded surface

Fig 6 Corrosion rate of various cemented carbide grades as a function of pH Source: Ref 23

As can be seen in Fig 6, straight WC-Co grades are resistant down to pH 7 This is also valid for WC-Co grades containing cubic carbides such as TiC, TaC, and NbC The highest corrosion resistance is obtained for certain alloyed TiC-Ni grades, which are resistant down to about pH 1, but compared to the straight WC-Co grades, they are less tough and have lower thermal conductivity They also have the disadvantages of being difficult to grind and braze; therefore, they are used only in specific applications

In many corrosion-wear situations, the proper choice is specially alloyed WC-Ni grades, which are resistant down to pH 2

to 3 Even in certain solutions with pH values less than 2, they have proved to be resistant to corrosion Because WC is the hard principal constituent and because nickel and cobalt are similar metals in many respects, their mechanical and thermal properties are comparable to those of the straight WC-Co grades

The pH value is one of the most important parameters when determining the corrosivity of a medium, but other factors such as temperature and electrical conductivity also have a great influence The latter is dependent on the ion concentration, that is, the amount of dissolved salts in the solution Thus, one cannot define the corrosivity of a certain medium in a simple way, and accordingly, no general rules that are valid in all situations can be given However, Table 6 gives general guidelines for the corrosion resistance of WC-Co and TiC-Ni cemented carbides in various room-temperature media Table 7 gives compatibility data for several types of cemented carbides in aqueous media at various temperatures, and Table 8 lists weight loss as a function of cobalt content for cemented carbides in mineral acids

Table 6 Corrosion resistance of cemented carbides in various media at room temperature

Corrosion resistance(a)

Medium

WC-Co cemented carbides

TiC-Ni cemented carbides

Organic solvents, including acetone, alcohols, gasoline, benzene, carbon tetrachloride, and ethylene glycol E E

(a) Corrosion resistance: E, excellent; V, very good; F, fair; G, good; P, poor

Table 7 Corrosion resistance of cemented carbides in various media

Data for two AISI austenitic stainless steels are included for comparison

Type of cemented carbide/corrosion resistance(a)

AISI stainless steels(b)

TiC-

WC-Ni

CoCr

TaC-

C(c)

C C A

Freon gas C 2 Cl 3 F 3 /CH 2 Cl 3 Room A A A A A

Oxalic acid (COOH) 2 ·2H 2 O 5 60 (140) 1 B-C A B A

Source: Ref 23, 24

(a) A, highly resistant, negligible attack; B, resistant, light attack; C, poor resistance, medium attack; D, not resistant, not suitable This table should

be used only as a guide Many factors, such as temperature variations, changes in chemical environment, purity of solutions, and stress or loading conditions, may invalidate these recommendations Tests under operating conditions should be made

(b) Results were obtained under laboratory conditions in pure solutions and are classified with reference to corrosion resistance only

(c) Coupled to brass

Table 8 Corrosion of WC-Co cemented carbides in mineral acids

Corrosion rates for AISI type 304 stainless steel are shown for comparison

Weight loss mg/mm2

37% HCl 5% HCl, 10% H 2 SO 4 5% H 2 SO 4 10% HNO 3 5% HNO 3

Room temperature

100 °C (212 °F)

100 °C (212 °F)

Room temperature

100 °C (212 °F)

Room temperature

100 °C (212 °F)

be chosen

Special Corrosion-Resistant Grades. To obtain corrosion resistance above and beyond that available with the regular WC-Co and TiC-Ni grades, the special corrosion-resistant grades are used These always result in a sacrifice in strength, hardness, and/or abrasion resistance, as shown in Table 2 On the other hand, the corrosion-resistant grades do offer significant benefits in corrosion resistance in many media (Table 7) These grades include the WC + Ni binder, the

WC + Co-Cr binder, and the so-called binderless WC, which generally contains about 10% TaC and between 1 and 2%

Co In addition, there are other special grades, such as the 0.1 to 1.0% Pt addition patented as an improvement toward ink corrosion resistance in ballpoint pen balls (Ref 26)

Sintered cemented carbide compositions based on more than 50% Cr2C3, for corrosion resistance are also mentioned in patents (Ref 27) and the literature (Ref 3, 4, 5) These are generally not commercially viable and are brittle materials; therefore, they cannot compete with the ceramic materials, such as silicon carbide, silicon nitride, aluminum oxide, boron nitride, and the whisker-reinforced ceramics, which have superb corrosion resistance Where impact and chipping are not

problems, these ceramic materials are a better choice than the cemented carbides The cemented carbides have the advantage, however, in strength, impact resistance, thermal conductivity, and often greater ease of manufacture

The best recent work showing the performance of the special corrosion-resistant compositions compared to the standard compositions and even some experimental compositions is that done at Metallwerk Plansee (Ref 7, 28) Table 9 lists the properties of these grades; for convenience, the proprietary designations are given, and the grades are also noted by composition, such as WC-10Co-4Cr Grades are also listed by a grade number that can be used when referring to Fig 7,

8, 9, 10, 11, 12, 13, and 14

Table 9 Properties of corrosion-resistant cemented carbide grades

See Fig 7, 8, 9, 10, 11, 12, 13, and 14 for the corrosion resistance of 12 of these grades in various media

WC Ni Co Cr

Others

HV (30-gf load)

Converted

to HRA

Density, g/cm3

Source: Ref 7 and 28

(a) Used to refer to grades in Fig 7, 8, 9, 10, 11, 12, 13, and 14

(b) Metallwerk Plansee grade designation

(c) Depending on the reference used, these grades are sometimes shown with small additions of TaC and TiC

(d) Kennametal designation

(e) Metallwerk Plansee experimental designation

Fig 7 Corrosion resistance of cemented carbides in 22% HCl at room temperature See Table 9 for properties

of these grades, and Fig 8, 9, 10, 11, 12, 13, and 14 for corrosion resistance in other media Source: Ref 7 and

28

Fig 8 Corrosion resistance of cemented carbides in 37.8% HNO3 at room temperature See Fig 7 for key to identification and compositions See also Table 9 and Fig 9, 10, 11, 12, 13, and 14 Source: Ref 7 and 28

Fig 9 Corrosion resistance of cemented carbides in 9.8% H2SO4 at room temperature See also Table 9 and Fig 7, 8, and 10, 11, 12, 13, and 14 Source: Ref 7 and 28

Fig 10 Corrosion resistance of cemented carbides in 6% acetic acid at room temperature See Fig 9 for key to

identification and compositions See also Table 9, Fig 7, 11, 12, 13, and 14 Source: Ref 7 and 28

Fig 11 Corrosion resistance of cemented carbides in 6.5% H3PO 4 at room temperature See Fig 9 for key to identification and compositions See also Table 9, Fig 7, 8, 9, 10, and Fig 12, 13, 14 Source: Ref 7 and 28

Fig 12 Corrosion resistance of cemented carbides in 4% NaOH at room temperature See Fig 9 for key to

identification and compositions See also Table 9, Fig 7, 8, 9, 10, 11, and Fig 12 and 13 Source: Ref 7 and 28

Fig 13 Corrosion resistance of cemented carbides in 2.9% NaCl at room temperature See Fig 9 for key to

identification and compositions See also Table 9, Fig 7, 8, 9, 10, 11, 12, and Fig 14 Source: Ref 7 and 28

Fig 14 Resistance to erosion-corrosion of cemented carbides in a room-temperature slurry of artificial

seawater and sand See Fig 9 for key to identification and compositions See also Table 9 and Fig 7, 8, 9, 10,

11, 12, and 13 Source: Ref 7 and 28

Figure 7 shows the corrosion of the 12 different compositions listed in Table 9 in 22% HCl at room temperature Grades 1

to 3 (WC-3Co, WC-6Co, and WC-9Co, respectively) illustrate the increase in corrosion rate that results from increasing cobalt binder content The nickel binders (grades 5 and 6; WC-6Ni and WC-9Ni, respectively) are an improvement, but again, the increase in binder content increases the corrosion rate Of the more exotic compositions, grades 4 (WC-10Co-4Cr) and 7 (WC-6NiCr) are viable choices for limited use in HCl at room temperature The best of the experimental compositions is grade 9 (WC-40TaC-3NiCoCr); it has greater strength and higher hardness If additional strength is needed above grade 9, grade 11 (WC-40TaC-9NiCoCr) is a good choice with the increases binder content, but as is generally the case, this results in a loss of corrosion resistance

Figure 8 shows the same type of information for 38% HNO3 at room temperature In general, corrosion is lower, but again, the higher-cobalt WC-Co compositions (grades 2 and 3) are not suitable, nor is the WC-9Ni composition (grade 6) Grade 5 (WC-6Ni) is marginal in HNO3, but grades 1 and 4 are still better On the other hand, the commercially available grades 7 and 8 (WC-6NiCr and WC-9NiCr, respectively) show very limited corrosion attack that is virtually equal to that

of three of the four experimental grades; the commercial alloys in this case have better strength

The basic cemented carbides are attacked most severely by H2SO4 (Fig 9) Some of the WC-Ni or WC-NiCr commercial compositions can tolerate limited use However, the experimental grade 7 (WC-40TaC-3NiCoCr) provides exceptional corrosion resistance

Figure 10 shows that many compositions are available for use in acetic acid with little corrosion Attack in H3PO4is relatively only on the WC-Co compositions(Fig 11)

Figures 12 and 13 show the suitability of all of the compositions listed in Table 9 in sodium hydroxide (NaOH) and sodium chloride (NaCl) In NaCl, there is significant benefit in choosing a nickel binder cemented carbide (for example, grade 5, WC-6Ni) if the loss in strength can be tolerated

Figure 14 shows the resistance to erosion-corrosion of different cemented carbide compositions in a slurry of artificial seawater and sand It follows the pattern of benefit for the use of nickel binders in saline applications The best of the WC-Co compositions is obviously the one with the lowest binder content (WC-3Co; grade 1) It shows a rate, however, more than 10 times greater than the experimental grade 9 (WC-40TaC-3NiCoCr), and both have the same transverse

rupture strength and equivalent hardness For a commercial composition, the grade 9 (WC-9NiCr) cemented carbide shows excellent performance, with one-half the rate of attack of the low-cobalt composition (grade 1, WC-3Co) and much higher transverse rupture strength

Some of the same data are shown in Fig 15 and 16 to compare the relative corrosion of the different compositions in various media These tests were performed at room temperature, and solution concentrations are the same as those in Fig

7, 8, 9, 10, 11, 12, 13, 14 In Fig 16, the cemented carbides are also compared to an Fe-20Cr-32Ni alloy; the superiority

of the experimental WC-40TaC-3NiCoCr cemented carbide is evident As with corrosion test data, care must be taken not

to extrapolate these to different solution concentrations and temperatures It would be logical to assume, for example, that the WC-40TaC-3NiCoCr alloy would always outperform WC-3Co in these media at different concentrations and temperatures, but the validity of this assumption must be verified through further testing

Fig 15 Corrosion resistance of four commercial cemented carbide compositions in aqueous media at room

temperature Source: Ref 28

Fig 16 Comparison of the corrosion resistance of a commercial WC-3Co cemented carbide and two

experimental compositions in aqueous media Source: Ref 28

Corrosion in Warm Acids and Bases. The corrosion rate of various cemented carbide compositions in warm (50 °C,

or 120 °F) acids is shown in Table 10 The straight WC-Co compositions show rapid attack in dilute H2SO4and HNO3, and little attack in those concentrated acids Although the corrosion rate is lower in HCl, it is obvious that these compositions are not suitable for use in warm or hot acid solutions The TiC-6.5Ni-5Mo composition is quite good in

H2SO4, moderately good in HCl, and very poor in HNO3 Several of the binderless compositions and the TaC-base cemented carbide show very acceptable corrosion resistance in these warm acids These results are to be expected, because the cobalt and nickel binders are completely soluble in these acids

Table 10 Weight losses of cemented carbides immersed in various acids at 50 °C (120 °F) for 72 h

The corrosion rates of various cemented carbides in basic solutions at 50 °C (120 °F) is quite a different matter, as shown

in Table 11 Although corrosion does proceed, it is slow enough to demonstrate the utility of even the WC-Co compositions in such applications as seal rings in these basic solutions

Table 11 Weight changes of cemented carbides immersed in NaOH, KOH, and NaOCl at 50 °C (120 °F) for 72 h

Table 12 Compositions and properties of galvanic corrosion test specimens

TiC-base cermet TiC-10TiN-2.5Mo 2 C-15Ni 91.5 1500 218

WC-NiCrMo alloy WC-3TiC-1.5(Cr 3 C 2 Mo 2 C)-15Ni 89.0 2100 305

Source: Ref 30

The apparatus used for the galvanic-corrosion testing is shown in Fig 17 Figure 18 shows the corrosion rates of the materials in the immersion test The binderless WC-3TiC-2TaC alloy performed the best, followed by the TiC-base cermet, the WC-Ni-CrMo alloy, the sintered cobalt-base alloy, and the WC-6Co alloy The logarithm of weight loss plotted against the logarithm of time yielded the linear weight loss curves in this test Based on this, it was postulated that the movement of electrons between cemented carbide and stainless steel is the rate-determining factor in galvanic corrosion Table 13 compares the corrosion rates of the materials in the immersion and galvanic tests For most of the alloy tested, the rate of galvanic corrosion is greater than the corrosion rate in the simple immersion test It is thought that the larger the potential difference between the cemented carbide and the stainless steel, the greater the difference between the corrosion rates obtained in the immersion test and in the galvanic-corrosion test

Table 13 Corrosion rates of immersion and galvanic corrosion test specimens

Weight loss, g/m2/d Specimen

Immersion test Galvanic test

Fig 17 Schematic of experimental apparatus used to study galvanic corrosion of cemented carbides in

seawater Source: Ref 30

Fig 18 Corrosion weight loss as a function of time for uncoupled test specimens from Ref 30

Figure 19 shows cross sections of specimen rings after the galvanic corrosion test Corrosion proceeded inward from the surface that contacted the seawater in the WC-6Co alloy (Fig 19a) The investigators postulated that the electrode potential is large and that electrons would move smoothly between the cemented carbide and the contacting stainless steel; therefore, attack proceeded according to the galvanic-corrosion mechanism In the case of the binderless WC-3TiC-2TaC alloy (Fig 19b), corrosion is very slight even after 1 year The TiC-base cermet (Fig 19c) shows corrosion only on the inner side surface (the side contacting the teflon; see Fig 17) In the sintered cobalt-base alloy and the WC-Ni-CrMo alloy (Fig 19c and d), corrosion proceeded from the corner that contacted both the seawater and the stainless steel It was postulated that the electrode potential and the distance of electron movement were smaller than those for the WC-6Co alloy Based on the results of these tests, either the binderless alloy or the TiC-base alloy should be acceptable for this type of application

Fig 19 Cross sections of galvanic-corrosion test specimens after (left to right) 1 month, 3 months, 6 months,

and 12 months (a) WC-6Co alloy (b) WC-3TiC-2TaC binderless alloy (c) TiC-base cermet (d) Sintered base alloy (e) WC-NiCrMo alloy Source: Ref 30

cobalt-Crevice Corrosion. The same investigators also reported on the crevice-corrosion resistance of cemented carbides in seawater with specimens of type 316 stainless steel, teflon, and silicon carbide adjacent to the cemented carbide specimens (Ref 30) Of the five compositions tested, only the WC-6Co specimen showed any significant attack after 1 year The attack was moderate and progressed the least against the silicon carbide and the most against the stainless steel (Ref 30)

Oxidation Resistance of Cemented Carbides

The ordinary WC-Co cemented carbides are reasonably resistant to oxidation in air up to about 650 to 700 °C (1200 to

1290 °F) The constituent affected the faster is WC, which will oxidize to WO3 In oxygen, the temperature limit is lower, and rapid deterioration will occur at about 500 °C (930 °F) Even in air, however, the practical temperature limit for WC-

Co compositions for any length of time is 500 to 600 °C (930 to 1110 °F) Nonetheless, these compositions do stand up, for example, in cutting tools in which localized higher temperatures at the cutting tip will be encountered The addition of both or either TiC or TaC to the WC-Co compositions increases the oxidation resistance somewhat and is undoubtedly also related to the improvement found for these additions for machining steel In applications in which oxidation resistance combined with wear resistance is required, as in hot glass forming and shearing tools, the addition of TiC and/or TaC to the basic WC-Co is of little benefit The TiC-Mo2C-Ni compositions have clearly superior oxidation resistance and can be used at temperatures up to 900 °C (1650 °F), at which point they start to oxidize fairly rapidly At the lower temperature, the TiC-base compositions form a tight adherent oxide film that tends to resist rapid attack This behavior difference is analogous to the behavior difference between cobalt and nickel alone, but WC is also more readily oxidized that TiC

Saw Tips and Corrosion

Cemented carbides are in widespread use in slitter saws, which are used to saw all types of metals, composites, lumber, and many other materials Small saw blades are sometimes manufactured from a single piece of carbide; larger blades, which may run up to 2 m (6 ft) in diameter, more commonly use cemented carbide tips brazed onto the steel saw body The heavy-duty chain saws used in the lumber industry also have carbide teeth Selection of cemented carbides for these

applications is invariably based on the need for excellent wear resistance and toughness Basic WC-Co compositions are almost always used

The rapid dulling of saws in such applications, however, is attributable to corrosive as well as abrasive conditions For example, on investigation studied the corrosion of WC cutting tools used to cut western red cedar (Ref 31) Tests were performed to determine the relative rates of attack of WC and cobalt in substance extracted from western red cedar, which has a higher content of such substances than other commercial lumber species Because the WC was not attacked, it was concluded that the cobalt binder content should be reduced to minimize attack Alternatively, the cobalt binder could be replaced with another binder material, such as nickel; however, such a substitution would result in a serious loss of strength Thus, the solution to this particular problem is not a simple one, and western red cedar is still being sawed primarily with WC-Co cemented carbide compositions

It was also suggested that the carbide be coated with TiC, TiN, or Al2O3 (or a combination of these) To date, these coatings are not used in such applications because of the need for resharpening and because of the difficulties of brazing a coated tip

Coating of Cemented Carbides

This widely used process has been primarily applied to metal cutting tools Certain special applications can be cited, such

as the coating of cemented WC watch cases with TiN to form a hard, corrosion-resistant gold-colored watch case (bezel) Clearly, the potential exists to utilize these thin (2 to 10 m, or 0.08 to 0.4 mil) coatings on wear- and corrosion-resistant parts The limitation is that the coating must be very thin to avoid spalling or chipping In addition, because the use of a cemented carbide in a corrosion application will only be in a very high-integrity, relatively costly application, the potential danger of a coating failing or being locally breached rules out consideration in most applications Despite this, coating of cemented carbides is an important state of the art that must be considered in special applications

Coating is most commonly done by chemical vapor deposition (CVD), and this process gives a wide range of possible coating materials In addition to the common TiN, TiC, Al2O3, perfectly feasible coating materials include hafnium carbide (HfC), hafnium nitride (HfN), zirconium carbide (ZrC), zirconium nitride (ZrN), TaC, and NbC The state of the art includes all combinations of TiC, TiN, Al2O3, and titanium carbonitride (TiCN), with limited commercial use of HfN and TaC as coating materials Chemical vapor deposition is generally performed at 900 to 1100 °C (1650 to 2010 °F) Titanium nitride is coated at lower temperatures, down to perhaps 700 °C (1290 °F), in less used commercial apparatus

Physical vapor deposition (PVD) has the advantage of being done at lower temperatures, down to perhaps 500 °C (930

°F), but it is a line-of-sight process that generally requires rotation of the parts being coated Deposition rates for PVD are much lower than those of CVD, and PVD equipment is more expensive Physical vapor deposited coatings have also been limited commercially to TiN, usually at thicknesses of 3 m (0.12 mil) or less More information on the corrosion and wear resistance of coatings applied by these methods is available in the article "CVD/PVD Coatings" in this Volume

Although there are few applications in which cemented carbides are used solely for corrosion resistance, it is essential to recognize the availability of the coated carbides Coatings of TiN, TiC, or Al2O3 can impart important corrosion and oxidation resistance to cemented carbides

Special Surface Treatments

Considerable work has been done to enhance the surface properties of cemented carbides (Ref 32, 33, 34), but it generally has been derived from surface modification processes developed for other metals These surface treatments include boriding, nitriding, and ion implantation Most of the treatments have been used to enhance resistance to wear, abrasion,

or erosion The benefits, if any, of such treatments in increasing resistance to oxidation and corrosion are not yet well documented Nevertheless, these processes may have potential in special applications The article "Surface Modification"

in this Volume contains information on the ion implantation and laser surface modification processes and their effects on the surface properties of metals

References

1 "Hardmetals Metallographic Determination of Microstructure," ISO 4499, International Organization for