NICKEL ALLOYS

23.3 NICKEL ALLOYS

23.3.1 General Behavior

Adding copper to nickel improves, to a moderate degree, resistance under reducing conditions (e.g., nonoxidizing acids, including hydrofl uoric acid). Also, in line with the resistance of copper to pitting, the tendency of nickel – copper alloys to form pits in seawater is less pronounced than for nickel, and pits tend to be shallow rather than deep. Above about 60 – 70 at.% Cu (62 – 72 wt.% Cu), the alloys lose the passive characteristics of nickel and tend to behave more like copper (see Section 6.8.1 ) retaining, however, appreciably improved resis- tance to impingement attack. Hence, 10 – 30% Ni, bal. Cu alloys (cupro - nickels) do not pit in stagnant seawater and are resistant to corrosion in rapidly fl owing seawater. Such alloys containing a few tenths to 1.75% Fe, which still further improves resistance to impingement attack, are used for seawater condenser tubes. The 70% Ni – Cu alloy (Monel), on the other hand, pits in stagnant sea- water and, in seawater, is best used under rapidly moving, aerated conditions that ensure uniform passivity. It does not pit under conditions that provide cathodic protection, such as when the alloy is coupled to a more active metal (e.g., iron).

Additions of chromium to nickel impart resistance to oxidizing conditions (e.g., HNO 3 and H 2 CrO 4 ) by supporting the passivation process. The critical minimum chromium content [4] obtained from critical current densities for anodic passivation in sulfuric acid is 14 wt.% Cr. These alloys are more sensitive

412 NICKEL AND NICKEL ALLOYS

exposed to stagnant seawater. Chromium also imparts to nickel improved resis- tance to oxidation at elevated temperatures. One important commercial alloy contains 20% Cr, 80% Ni (see Section 11.13.4 ).

Alloying nickel with molybdenum provides, to a marked degree, improved resistance under reducing conditions, such as in hydrochloric acid. The corrosion potentials of such alloys in acids, whether aerated or deaerated, tend to be more active than their Flade potentials [5] ; hence, the alloys are not passive in the sense of Defi nition 1 (Section 6.1 ). For example, the corrosion potentials of nickel alloys

containing 3 – 22.8% Mo in 5% H 2 SO 4 , hydrogen - saturated, all lie within 2 mV of a platinized platinum electrode in the same solution [5] . Notwithstanding an active corrosion potential, the alloy containing 15% Mo, for example, corrodes at 1/12 the rate of nickel in deaerated 10% HCl at 70 ° C, and the rate decreases still further with increasing molybdenum content (1/100 the rate for nickel at 25% Mo). Polarization measurements show that molybdenum alloyed with nickel has little effect on hydrogen overpotential, but increases anodic polarization instead; hence, the corrosion rate of the alloy is anodically controlled. The mecha- nism of increased anodic polarization is related most likely to a sluggish hydra- tion of metal ions imparted by molybdenum, or alternatively to a porous diffusion - barrier fi lm of molybdenum oxide, rather than to formation of a passive fi lm typical of chromium or the passive chromium – nickel alloys.

As an alloying element in nickel, tungsten behaves similarly to molybdenum, but is less effective [6] . Because the binary nickel – molybdenum alloys have poor physical properties (low ductility, poor workability), other elements, for example, iron, are added to form ternary or multicomponent alloys. These are also diffi cult to work, but they mark an improvement over the binary alloys. Resistance of such alloys to hydro- chloric and sulfuric acids is better than that of nickel, but it is not improved with

respect to oxidizing media (e.g., HNO 3 ). Since the Ni – Mo – Fe alloys have active corrosion potentials and do not, therefore, establish passive – active cells, they do not pit in the strong acid media to which they are usually exposed in practice.

By alloying nickel with both molybdenum and chromium, an alloy is obtained resistant to oxidizing media imparted by alloyed chromium, as well as to reducing media imparted by molybdenum. One such alloy, which also contains a few percent iron and tungsten (Alloy C), is immune to pitting and crevice corrosion in seawater (10 - year exposure) and does not tarnish appreciably when exposed to marine atmospheres. Alloys of this kind, however, despite improved resistance to Cl − , corrode more rapidly in hydrochloric acid than do the nickel – molybdenum

alloys that do not contain chromium. Nominal compositions of some commercial nickel - base alloys containing copper, molybdenum, or chromium are given in Table 23.3 . The Ni – Cu alloys are readily rolled and fabricated, whereas the Ni – Cr alloys are less readily, and the Ni – Mo – Fe and Ni – Mo – Cr alloys are diffi cult to work or fabricate.

Some commercial Cr – Ni – Fe – Mo alloys corresponding in composition to high - nickel stainless steels also contain a few percent copper. They are designed to resist, among other media, sulfuric acid over a wide range of concentrations.

NICKE

T A B L E 23.3. Typical Compositions (%) of Commercial Nickel - Base Alloys

L ALL

Alloy Common

Mn a a C Other

OY

System Name of

No.

Alloy Ni – Cu

2.5 a 0.5 0.2 0.3 Cu: 31.5 Ni – Cr

5.0 a 0.5 0.5 0.1 Nb + Ta: 3.6 Al: 0.4; Ti: 0.4

Ni – Mo

B N10001

28 1.0 1.0 a 0.5 a 2.0 a 0.1 1.0 0.02 Cu: 0.5 a B-3

b 65 28.5 3.0 1.5 3.0 a 1.5 0.1 3.0 0.01 Cu: 0.2 a ; Al: 0.5 a Ni – Cr – Fe

N10675

8 0.5 0.5 0.15 Cu: 0.2 800H

0.5 0.08 Al: 0.4; Ti: 0.38 690

10 0.5 0.5 0.05 Cu: 0.5 a Ni – Cr – Fe –

6.5 2.5 22.0 1.0 a 19.5 1.0 2.0 0.05 Cu: 2.0; Nb: 2.0 Mo – Cu

G N06007

16 2.5 16 4 6 1.0 1.0 0.08 a V: 0.35 C - 22

C N10002

Bal.

a V: 0.35 C - 276

16 2.5 16 4 5 0.08 1.0 0.02 a V: 0.35

1.5 a 0.1 0.5 0.01 Al: 0.25 Ni – Fe – Cr

22.0 b 0.2 0.5 0.05 Al: 0.2 a ; Ti: 0.9 a Maximum.

b Minimum. Source:

Adapted from Metals Handbook, Desk Edition, 2nd edition, ASM International, Materials Park, OH, 1998, p. 610. For detailed alloy specifi cations, see, for example, Worldwide Guide to Equivalent Nonferrous Metals and Alloys, 4th edition, ASM International, Materials Park, OH, 2001.

414 NICKEL AND NICKEL ALLOYS

The function of alloyed copper is the same as that of alloyed palladium in tita- nium mentioned in Section 6.4 , namely, to accelerate the cathodic reaction (H +

or O 2 reduction) to the point where the anodic current density reaches or exceeds the critical value for anodic passivation.

23.3.2 Ni–Cu System: Alloy 400 —70% Ni, 30% Cu

Because nickel and copper in all proportions form solid solutions, many nickel – copper and copper – nickel alloys are possible. The fi rst commercial nickel – copper

alloy was Alloy 400 (commonly known as Monel), which is still widely used today. With about 31% copper in a nickel matrix, its corrosion behavior is similar to that of nickel in many ways, but improved relative to nickel in other ways [7] . Since Alloy 400 is resistant to high - velocity seawater, it is often used for valve trim and pump shafts. It is also used for industrial hot freshwater tanks and for equipment in the chemical industry. It resists boiling sulfuric acid in concentrations less than 20%, with the corrosion rate being < 0.20 mm/y ( < 0.008 ipy)(23 - h test) [8] . It is

outstandingly resistant to unaerated HF at all concentrations and temperatures up to boiling. [Rate in N 2 - saturated 35% HF, 120 ° C (248 ° F) is 0.025 mm/y (0.001 ipy); when air saturated, 3.8 mm/y (0.15 ipy) [9] .] Resistance to alkalies is good, except in hot concentrated caustic solutions or aerated NH 4 OH. Alloy 400 is not resistant to oxidizing media (e.g., HNO 3 , FeCl 3 , CuSO 4 , and

H 2 CrO 4 ) nor to wet Cl 2 , Br 2 , SO 2 , and NH 3 . The alloy is susceptible to S.C.C. in moist aerated hydrofl uoric acid and in hydrofl uorosilicic acid vapor, but suscep- tibility can be minimized by deaeration of the environment and by stress relief annealing the alloy component [10] .

23.3.3 Ni–Cr–Fe System: Alloy 600 —76% Ni, 16% Cr, 7% Fe

Alloy 600 resists oxidizing aqueous media [e.g., mine waters, Fe 2 (SO 4 ) 3 , CuSO 4 , and HNO 3 ]. In nitric acid, resistance is best above 20% HNO 3 , including red fuming acid, but is not as good, in general, as the stainless steels. Resistance to alkalies is good, except in concentrated hot caustic solutions. It is also resistant

to all concentrations of NH 4 OH at room temperature. Like the stainless steels, this alloy tends to pit in seawater and also in oxidizing metal chlorides, such as FeCl 3 . Alloy 600 is used extensively where oxidation resistance at elevated tempera- tures is required (see Section 11.13.4 ). Pressurized water reactors of nuclear - power plants use mill - annealed Alloy 600 tubes for heat exchangers. Typically, reactor coolant circulates through the tubes at 315 ° C (600 ° F) exiting 30 – 35 ° C lower in temperature. Boiler water in contact with the outside of the tubes is all - volatile treated (minimum dissolved

solids and dissolved O 2 , slightly alkaline using NH 3 ). Thinning and intergranular S.C.C. of tubes tend to occur at inlet - end sections above the tube sheet in crevice and sludge buildup areas [11] . Washings from such areas test alkaline and are high in sodium. Hence, laboratory tests of stressed alloy exposed to hot NaOH

NICKEL ALLOYS

solutions (290 – 365 ° C) are used as an accelerated measure of resistance to S.C.C. in operational steam units. Such tests show that heat treatment of Alloy 600 at either 650 ° C for 4 h or at 700 ° C for 16 h or more considerably improves resistance to S.C.C. in NaOH solutions [12] . The improved resistance also extends to an environment of relatively pure water at 345 ° C (650 ° F) [12] . Through heat treat- ment, discontinuous carbide precipitates form at grain boundaries accompanied at 650 ° C (but not at 700 ° C) by chromium depletion. The latter situation accounts

for intergranular corrosion of the alloy by 25% HNO 3 and by polythionic acids. But whether or not chromium depletion occurs is not related to the cause of intergranular S.C.C.

Accordingly, Alloy 600 fails in 10% NaOH at 315 ° C (600 ° F) independent of carbon content (0.006 – 0.046%) [13] ; and an alloy (18% Cr, 77% Ni) approximat- ing the Alloy 600 composition, but containing only 0.002% C, remains susceptible in water at 350 ° C (660 ° F) [14] . The long incubation time typically required for cracks to initiate in pure water (several months) supports the view that specifi - cally damaging elements must reach a critical concentration by slow diffusion to grain boundaries in order for the alloy to become susceptible. Both phosphorus and boron have been suspected [13, 15] (see also Section 19.2.3.3 ). Alloy 690, however, has been found to provide satisfactory service, with no reported failures by intergranular S.C.C. [16] .

Alloy 600 is resistant to S.C.C. in boiling MgCl 2 solutions. The extent of S.C.C. of the alloy in water at 350 ° C is unaffected by, and remains intergranular in, the presence of 0.1% NaCl [14] .

23.3.4 Ni–Mo System: Alloy B —60% Ni, 30% Mo, 5% Fe

The original Alloy B (N10001) was invented in the 1920s. Alloy B and commercial alloys of similar composition are resistant to hydrochloric acid of all concentra- tions and temperatures up to the boiling point. The rate of corrosion is 0.23 mm/y (0.009 ipy) in boiling 10% HCl; 0.5 mm/y (0.02 ipy) in boiling 20% HCl; and

0.05 mm/y (0.002 ipy) in 37% HCl at 65 ° C (150 ° F) [17] . Resistance to boiling sulfuric acid is good up to 60% H 2 SO 4 < 0.2 mm/y, < 0.007 ipy). In phosphoric acid, ( rate of attack is low for all concentrations and temperatures, with the highest rate for the pure acid applying to the boiling 86% H 3 PO 4 (0.8 mm/y, 0.03 ipy). In various organic acids, hot or cold, resistance is also good. Because of the lack of chromium, Alloy B is not resistant to oxidizing condi- tions (for example, HNO 3 ) or to oxidizing metal chlorides (such as FeCl 3 ) [18] . Contamination of nonoxidizing acids with oxidizing ions (such as Fe 3+ or Cu 2+ ) causes a large increase in corrosion rate [19] . Nickel – molybdenum alloys must not be used where oxidizing conditions exist.

Because of the high carbon content of Alloy B, it is subject to intergranular attack in nonoxidizing acids (e.g., acetic, formic, and hydrochloric) when heated in the range 500 – 700 ° C (930 – 1300 ° F), as may occur in the heat - affected zone (HAZ) of a weld. Heat treatment to avoid such attack consists of annealing at 1150 – 1175 ° C (2100 – 2150 ° F), followed by rapid cooling in air or water. The

416 NICKEL AND NICKEL ALLOYS

low - carbon alloy, B - 2, is less susceptible to intergranular attack and HAZ corro- sion, but it is diffi cult to fabricate and can crack during manufacturing operations. These embrittlement problems were resolved by developing Alloy B - 3, with care- fully controlled additions of iron, chromium, and manganese [18, 19] .

23.3.5 Ni–Cr–Fe–Mo–Cu System: Alloy G—Ni, 22% Cr, 20% Fe, 6.5% Mo, 2% Cu

The high nickel content in these alloys provides corrosion resistance in reducing environments. The chromium content contributes to strength, aqueous corrosion resistance in oxidizing environments, and oxidation resistance. Alloy G was developed during the 1960s. Alloy G - 3 is an improvement on Alloy G, but with greater resistance to HAZ attack and better weldability. The lower carbon content results in slower kinetics of carbide precipitation, and the higher molybdenum content improves the localized corrosion resistance. As a result, Alloy G - 3 has replaced Alloy G in most applications. Alloy G - 3 is widely used for downhole tubulars in the oil and gas industry [20, 21] .

23.3.6 Ni–Cr–Mo System: Alloy C —54% Ni, 15% Cr, 16% Mo, 4% W, 5% Fe

The Ni – Cr – Mo alloys are widely used within the chemical process industry. These alloys are resistant to both oxidizing and reducing environments. They resist chloride - induced pitting, crevice corrosion, and S.C.C. In addition, they are easily formed and are weldable.

Because of its chromium content, Alloy C is resistant to such oxidizing media as HNO 3 , HNO 3 –H 2 SO 4 mixed acids, H 2 CrO 4 , Fe 2 (SO 4 ) 3 , FeCl 3 , and CuCl 2 . Cor- rosion rates fall below 0.5 mm/y (0.02 ipy) in < 50% HNO 3 at 65 ° C (150 ° F), but are higher above this concentration. In boiling > 15% HNO 3 , rates are high. Resistance is excellent to wet or dry Cl 2 at room temperature; pitting in wet Cl 2 may occur at higher temperatures. Alloy C also resists wet or dry SO 2 up to about

70 ° C (155 ° F) [17] . Alloy C has outstanding resistance to pitting or crevice corro- sion in seawater. Resistance to acetic acid is excellent. In boiling 40% formic acid, the rate is 0.25 mm/y (0.01 ipy).

Alloy C has good resistance to hydrochloric acid at room temperature [0.025 mm/y (0.001 ipy) for 37% HCl], but not at higher temperatures [5 mm/y (0.2 ipy) for 20% HCl, 65 ° C (150 ° F)]. It is recommended for boiling sulfuric acid up to 10% acid (0.25 mm/y) and for boiling phosphoric acid up to at least 50% acid (0.25 mm/y).

When heated in the temperature range 500 – 700 ° C (930 – 1300 ° F), the alloy tends to corrode intergranularly; for this reason, weld HAZs undergo severe intergranular corrosion, which can be avoided by a fi nal heat treatment at 1210 –

1240 ° C (2210 – 2260 ° F), followed by rapid cooling in air or water [22] . Alloy C - 276, introduced in the 1960s, contains less carbon and is less sensitive to intergranular corrosion resulting from intermediate heat treatment. For this

REFERENCES

reason, Alloy C - 276 can be used in most applications in the as - welded condition without severe intergranular attack.

Alloy C - 22 and Alloy 59 were developed for applications where resistance to corrosion under highly oxidizing conditions is required. The high chromium content in these alloys, 22% and 23% for C - 22 and 59, respectively, imparts excel- lent corrosion resistance of these alloys to nitric acid [23] . In addition, these alloys have been found to have superior crevice corrosion resistance in seawater [24] .

23.3.7 Ni–Fe–Cr System: Alloy 825 —Ni, 31% Fe, 22% Cr

The Ni – Fe – Cr system bridges the gap between the high - nickel austenitic stainless steels and the Ni – Cr – Fe alloy system. Alloy 825 is an upgrade from the 300 series stainless steels and is used to obtain resistance to localized corrosion and S.C.C. This alloy was developed from Alloy 800 by increasing the nickel content and by adding molybdenum (3%), copper (2%), and titanium (0.9%) for improved cor- rosion resistance in many media [20, 21] .