COBALT ALLOYS

24.2 COBALT ALLOYS

Being less plentiful and more expensive than nickel, cobalt is usually alloyed with chromium for applications where the alloys have practical advantages over similar nickel - or iron - base alloys. The cobalt - base alloys, for example, are better resistant to fretting corrosion, to erosion by high - velocity liquids, and to cavita- tion damage.

Chromium - bearing cobalt alloys can be divided into three groups [2] :

1. Alloys for wear resistance

2. Alloys for high - temperature use

3. Alloys for resistance to aqueous corrosion and wear Cobalt alloys were developed in the early 1900s by Elwood Haynes of

Kokomo, Indiana, for resisting corrosive environments and for high - temperature strength and hardness. The alloys are used in cutting tools operating in aggressive chemical media, steam valves and valve seats, pressure gauges, bushings, nozzles, pressure seats, and implant materials for the human body.

At temperatures up to 417 ° C, cobalt has a hexagonal close packed (hcp) structure, whereas above 417 ° C, it is face - centered cubic (fcc). Alloying elements that are fcc stabilizers (such as nickel, iron, and carbon) lower the transformation

COBALT ALLOYS

temperature. On the other hand, chromium, molybdenum, and tungsten, hcp stabilizers, raise the transformation temperature. Alloys of cobalt that contain both fcc and hcp stabilizers have transformation temperatures that are a complex function of the alloy composition [3] . Applying stress to alloys in a metastable fcc structure at ambient temperature can partially transform them to hcp.

Cobalt can be anodically passivated in 1 N H 2 SO 4 , with the required minimum current density of 5000 A/m 2 (500 mA/cm 2 ) being 14 times higher than that for nickel [4] . Alloying cobalt with chromium reduces the current density, with the 10% Cr alloy requiring only 10 A/m 2 (1 mA/cm 2 ) to become passive. The alloy containing 10 – 12% Cr is negligibly attacked by 10% HNO 3 , hot or cold; but in 10% H 2 SO 4 or HCl, passivity is lost, and corrosion rates become excessively high. Alloying of Co – Cr alloys with molybdenum or tungsten reduces attack by H 2 SO 4

or HCl, but not by HNO 3 .

Some commercial cobalt - alloy compositions are listed in Table 24.1 . They typically contain chromium as well as molybdenum and/or tungsten, making them relatively resistant to both reducing and oxidizing conditions. High resis- tance to abrasion results from the precipitation of carbides, which can be carbides of chromium, molybdenum, and tungsten [3] .

The alloys resist pitting in FeCl 3 at room temperature, except Alloy 6B because of its high carbon content of 1.2%. At a lower carbon content — for example, 0.4% — the alloy is resistant. Vitallium, having a very noble critical pitting potential is also resistant to pitting in dilute NaCl solutions, a property that extends to Alloy 25 and MP35N as well.

Substantial additions of cobalt to chromium plus molybdenum (or tungsten) alloys are detrimental to S.C.C. behavior, resembling the effect of added iron rather than the benefi cial effect of added nickel. Accordingly, the MP35N alloy

resists S.C.C. in MgCl 2 solution at 153 – 154 ° C, but by replacing most nickel with cobalt (and the incidental reduction of molybdenum to 6% and increase of chro- mium to 30%), as in Vitallium, susceptibility results [5] . Alloy 25 is also suscep- tible. This susceptibility does not include Vitallium exposed to saline solutions at 37 ° C (body temperature) in which the alloy is resistant. The situation is analo- gous to the observed resistance of 18 – 8 (types 304 and 316) stainless steels to S.C.C. in aerated chlorides at temperatures below 60 – 80 ° C, but not above.

In boiling 50% NaOH, all of the stressed cobalt alloys fail by S.C.C., or sometimes by relatively rapid uniform dissolution. When severely cold - worked and cathodically polarized at room temperature in 5% H 2 SO 4 plus As 2 O 3 , the stressed alloys are susceptible to failure by hydrogen cracking. Similar failures are also expected when the alloys are coupled in the same acid to a more active metal like iron, with the resultant galvanic action paralleling cathodic polariza-

tion. Failures in 5% NaCl – 0.1% acetic acid, saturated with H 2 S (NACE solution [6] ), which simulates deep sour gas well environments, depend on temperature and the prevailing uniform corrosion rate to produce hydrogen. At room tem- perature, failures in this solution by hydrogen cracking (also sometimes called sulfi de stress cracking) usually occur only for the cold - worked alloys that are subsequently heat - treated. This heat treatment improves strength, but may also

T A B L E 24.1. Nominal Compositions (%) of Cobalt Alloys Alloys for

Co Other Name

30 1.5 a 4 2.5 a 3 1 1.4 0.7 bal. — resistance

Wear

6B R30016

15 10 a 3 0.1 1.5 0.4 a bal. — temperatures

2.75 a 3 0.25 a 1 a 1 bal. a B 0.007 corrosion +

35 a 1 0.025 a 0.15 a 0.15 a bal. Ti 1 a wear

MP35N

26 5 2 9 3 0.06 0.8 0.3 bal. N 0.08 a a resistance CO Vitallium — 30 6 — — — 0.5 0.75 — bal. —

BA a Maximum.

L T Source:

Data from ASM Handbook , Vol. 13B, Corrosion: Materials , ASM International, Materials Park, OH, 2005, pp. 165, 172. AN

D CO BA

L T A L LO

GENER AL REFERENCES

423

increase uniform corrosion rates in acids suffi cient to generate hydrogen in amounts necessary to induce cracking. Failure by hydrogen cracking usually diminishes as the temperature is raised (less hydrogen enters the metal and more escapes as gas), but chloride S.C.C. may displace hydrogen cracking as the failure mechanism in the higher - temperature range. In this event, coupling of the alloys to a more active metal, as in cathodic protection, prevents cracking.

The advantage of cobalt - base alloys to reduce fretting corrosion contributes to the advantage of using Vitallium in the human body. Waterhouse [7] showed that a Vitallium screw mounted in a metal plate in saline solution and subjected to variable stress so as to cause slight rubbing of the screw head was damaged less than stainless steel. Also, in laboratory cavitation - erosion tests in distilled water, cobalt alloys exhibited superior resistance [8] . Vitallium and Alloys 6B and 25 lost only 1/3 to 1/14 the weight loss of similar specimens of nickel - base C - 276 and iron - base type 304 stainless steel. Similarly, in high - velocity (244 m/s) hot brines common to geothermal wells, Alloy 25 and MP35N resisted corrosion - erosion better than C - 276 and much better than 26% Cr – 1% Mo stainless steel [9] .