Coriou reported IGSCC (PWSCC) susceptibility for nickel-based alloys and the influence of nickel con­tent in high-temperature, high-purity water.18 It is now known, however, that this cracking susceptibility

Table 15 Corrosion rate of nickel and nickel-based alloys in liquid sodium hydroxide (mmyear-1) Alloy Testing temperature (°C) 400 500 580 680 201 0.02 0.03 0.06 0.94 C — 2.56 a — 400 0.05 0.13 0.45 — 600 0.03 0.06 0.13 1.69 ‘Surface was swelled by oxides.

Table 16 Maximum applicable temperature (°C) in dried HCl and Cl2 gas Alloy or steel HCl Cl2 Alloy 201 510 538 Alloy 600 482 538 Alloy B 454 538 Alloy C 454 510 Alloy 400 232 427 316 stainless steel 427 343 304 stainless steel 399 316 Carbon steel 260 204

is very dependent on the corrosion potential, as determined by the concentration of molecular hydro­gen in solution (as in PWR primary water). Subse­quently, it was reported that IGSCC is affected by the chromium content, but not by the nickel content in nickel-based alloys, as shown in Figures 23 and 24 60 Alloy 690 has higher resistance to PWSCC than Alloy 600, due to its higher chromium content.

Carbide precipitation along grain boundaries by thermal treatment (TT) at around 700 °C improves the PWSCC resistance for Alloys 600 and 690. In particular, M23C6 precipitation that is coherent with the matrix was detected along grain boundaries in the TT Alloy 690, which has excellent PWSCC resistance depending on the carbon content and the solution heat-treatment temperature. By contrast, niobium addition to Alloy 600 was found to have a poor effect on PWSCC susceptibility,60 but improves IGSCC resistance under BWR water conditions.25

2.08.4.2 In High-Temperature Gases

2.08.4.6.1 Oxidation

Nickel shows superior oxidation resistance to carbon steels and copper alloys due to the formation of a

1000

ЕЕГ

0

500

Ф

3

Q.

3

DC 100

50

70 80 90

Nickel content (%)

Figure 24 Effect of Ni content on the stress corrosion cracking fracture time under constant load test for solution-annealed nickel-based 5% chromium-iron alloys at 360 °C in simulated pressurized water reactor primary water.

nickel oxide film in air or other oxidizing environ­ments. The oxidation resistance of nickel is improved remarkably by the addition of chromium. Repeated oxidation test results for various alloys are shown in Figures 25 and 26.10,11,14 In these tests, Alloy 600 showed little weight change and was found to have better oxidation resistance than 304 or 310 stainless steels. Alloy 601 had higher oxidation resistance than Alloy 600, due to its higher chromium and aluminum contents. Alloy 690 also had higher oxidation resis­tance than Alloy 600 due to its higher chromium content.

2.08.4.6.2 Nitriding

Low-alloy steels are highly susceptible to nitriding in active atmospheres such as high-temperature ammo­nia gas. To obtain resistance to nitriding, the addition of nickel is effective. Austenitic stainless steels have higher resistance to nitriding than low-alloy steels, for example. Nickel-based alloys have significantly better resistance to nitriding. Alloy 600 shows excel­lent resistance to nitriding in ammonia production plant environments.

2.08.4.6.3 Sulfidation

Nickel-based alloys are highly susceptible to sulfi­dation. Nickel forms a eutectic with sulfur at

C — heavy oil

(200 ppmV, 50 ppmNa, 2.5% S)
Temperature : 816 °C

a : 4500 h b : 9429h

c : 6450 h d : 1200h [11]

temperatures above 645 °C and the scales on nickel lose their protective properties at higher tempera­tures. The addition of chromium to nickel-based alloys is effective for improving sulfidation resistance, and alloys containing higher than 20% chromium show good sulfidation resistance.

Nickel undergoes severe corrosion in combustion gases ofcrude petroleum. When vanadium is present in these gases, corrosion occurs due to the formation of low-temperature-melting compounds with vana­dium oxides (so-called vanadium attack). When sul­fur is present in crude petroleum, sulfide corrosion occurs.

50% chromium-50% nickel and 60% chromium — 40% nickel alloys are rare nickel-based materials with excellent resistance to vanadium attack and sulfide corrosion. Figure 2761 shows exposure test results for the supports of the super heater tubes of a fossil-fuel electric power plant. The data indicate that both alloys are indeed highly resistant to corrosion.

2.08.3 Summary

The excellent corrosion resistance and mechanical characteristics of various nickel-based alloys have been described in this chapter. In particular, typical corrosion, mechanical, other physical properties data, together with general fabrication information, have been reviewed.

Copper-based alloys have more than 5000 years of history, and iron-based alloys more than 4000 years. However, nickel-based alloys were developed only in the last 100 years or so. This very short history for nickel-based alloys means that some unknown, or uncertain, or unexpected scientific properties will be still remaining to be discovered for these alloys. Consequently, continuing and assiduous studies of nickel based alloys are plainly required (see Chapter

4.4, Radiation Effects in Nickel-Based Alloys and Chapter 5.04, Corrosion and Stress Corrosion Cracking of Ni-Base Alloys).