(5) Nickel Alloys
Nickel is an expensive metal but its alloys exhibit excellent anti-corrosion characteristics not seen with other metals in severe corrosive environments.
Monel Metal
A nickel alloy with 30% copper content, able to withstand high speed salt water flow, used for valve mating sections and pump shafts. It withstands hydrofluoric acid unlike other metals, but not good in oxidizing environments such as with nitric acid and chromic acid.
Inconel 600
A nickel alloy with 16% chromium and 7% iron, highly resistant to oxidation in high temperature and oxidizing solutions.
Inconel 626
A nickel alloy with 20~23% chromium and 8~10% molybdenum, shows higher corrosion resistance than Inconel 600 in oxidizing solutions. This alloy does not suffer from weaknesses of austenitic stainless steels such as pitting corrosion, crevice corrosion, and stress corrosion fractures.
Hastelloy C
A nickel alloy with 15% chromium, 16% molybdenum, and small amounts of tungsten and iron, similar in high corrosion resistance as Inconel but with susceptibility to inter-granular corrosion. Hastelloy C276 is an alloy with the carbon content reduced to below 0.02% to combat the aforementioned problem, as well as not suffering from pitting corrosion, crevice corrosion, and stress corrosion fractures, similar to Inconel 625.
Hastelloy B
A nickel alloy with 30% molybdenum and 5% iron. As opposed to Inconel 625 and Hastelloy C267 which are not strongly resistant to non-oxidizing acids such as hydrochloric acid and sulfuric acid, this alloy can withstand hydrochloric acid at any temperature as well as up to 60% boiling sulfuric acid. However, it is not resistant in oxidizing environment such as nitric acid.
(6) Titanium and Titanium Alloys
Titanium is generally regarded as omnipotently corrosion resistant, and is indeed very corrosion resistant in sea water not suffering from pitting corrosion or crevice corrosion. It withstands chlorine gas environments with water that other metals cannot cope with, and is resistant to nitric acid at any temperature.
However, since this metal owes its corrosion resistance to its passive surface layer, it is prone to suffer from crevice corrosion in environments with high concentrations of chlorides such as hydrochloric acid and sulfuric acid. To supplement the easy formation of the passive layer, there are alloys with 0.2% palladium and small amounts of nickel and molybdenum added.
High tensile titanium alloys are being developed to improve the mechanical strength. A representative of such is "6-4 Alloy" containing 6% palladium and 4% vanadium. This alloy, however, is not as corrosion resistant compared to pure titanium and results in stress corrosion where salts exist.
