How nickel alloyed with other elements improves the application experience

Nickel alloyed with molybdenum offers remarkable increase in resistance to reducing conditions for example HCl. The corrosion potentials of these alloys in acidic media both aerated and deaerated end to be more active than their Flade potentials, thus the alloys are not passive. For instance, the corrosion potentials of nickel based alloys containing 3 to 22.8% Molybdenum in 5% sulfuric acid concentration, hydrogen saturated, all lie within 2mV of a platinized platinum electrode in the same solution. Failed against active corrosion potential, the alloy with 15% molybdenum for instance is corroded at 1/12^th rate of nickel in deaerated 10% HCl at 70oC and the rate reduces further with increasing molybdenum concentration.

Molybdenum alloyed with nickel has nominal effect on hydrogen overpotential however increases anodic polarization thus the corrosion rate of alloy is anodically controlled. The method of increased anodic polarization is related most likely to a sluggish hydration of metal ions released by molybdenum or to a porous diffusion barrier layer of molybdenum oxide instead the development of a passive layer typically of chromium or the passive chromium-nickel alloys.

As an alloying element in nickel, tungsten acts similarly to molybdenum however it is less effective. Because Ni-Mo alloys have inadequate physical characteristics such as low ductility and low workability, other elements such as iron is added to develop multi element alloys. These are hard to work however they offer a significant improvement. These alloys offer better corrosion resistance against HCl and H2SO4 acids than nickel however it is not enhanced with respect to oxidizing media for example HNO3. As the NiMoFe alloys have active corrosion potentials and hence they do not create passive-active cells and they resist pitting in strong acid media.

Alloying nickel with molybdenum and chromium, an alloy is received that is resistant to oxidizing conditions offered by chromium element and reducing conditions with the contribution of molybdenum. An example of such alloy that also comprises of iron and tungsten is Hastelloy C276 bar resistant to pitting and crevice corrosion in sea water that even doesn’t tarnish noticeably when exposed to seawater conditions.

Some commercial Chromium – Nickel – Iron – Molybdenum alloys corresponding to composition to high nickel stainless steels contain some content of copper. They are made to prevent corrosion against sulfuric acid to its any concentration level. The performance of alloyed copper is identical to alloyed palladium in titanium to accelerate the cathodic reaction to the level where the anodic current density reaches or exceeds the critical value for anodic passivation.

Nickel-Copper alloy system

As nickel and copper in all concentrations develop solid solutions, production of various nickel-copper and copper-nickel alloys is feasible. A common and highly popular nickel-copper alloy is Monel alloy 400 that is widely used in the modern industries. Containing 31% copper, it offers similar corrosion resistance as of nickel in various manners however better than nickel in some ways. As alloy 400 prevents corrosion in high speed seawater, it is commonly used for valve trim and pump shafts.