EFFECT OF ALLOYING ELEMENTS

The corrosion behavior of the alloying elements in pure form influences the corrosion properties of the alloys made from them. Chromium is the first example, with outstanding corrosion resistance in the passive state.

In solutions of neutral pH, dissolved oxygen from the air is sufficient to maintain passivity. But in low-pH solutions, stronger oxidizing agents must be present and halogen or sulfuric acids absent in order to stabilize the passive condition. Chromium metal is not resistant to corrosion by reducing acids.

Some examples, from Uhlig, of corrosion resistance of electrodeposited chromium are as follows:

Three points can be made from these data. First, chromium as an alloying element is not particularly effective in promoting resistance to reducing or halogen acids. Second, in solutions of some halogen salts the passive layer was maintained by oxygen dissolved in the solution. Third, sulfuric acid behaves as a reducing acid in lower concentrations but as an oxidizing acid in concentrated form. When selecting alloys to resist sulfuric acid, one must bear this in mind. Stainless steels containing only chromium and iron, specifically the ferritic and martensitic stainless steels, likewise have poor resistance to sulfuric acid solutions but may resist nitric acid. These chromium–iron alloys are not resistant to corrosion by halogen acids or chloride salts. Those ferritic alloys that do have good to excellent chloride pitting resistance, such as E-Brite and AL29-4C , gain that resistance by the addition of 1 and 4% Mo, respectively. 

Austenitic nickel alloys with resistance to concentrated (oxidizing) sulfuric acid require high chromium, such as the ThyssenKrupp VDM alloy 33, or silicon, as in Haynes nickel alloy D-205, and the stainless grades A610, A611, and Sandvik SX. There are a few highly corrosion resistant nickel alloys with little or no chromium, including the various Ni–Mo ‘‘B’’ grades and the 67Ni–31Cu alloy 400. Excellent in reducing environments, they have almost no tolerance to oxidizing compounds in the environment. A newly developed nickel–molybdenum alloy, B-10, includes 8% Cr in its composition for limited resistance to low levels of oxidizers.

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STAINLESS STEELS
James Kelly
Rochester, Michigan

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
Edited by Myer Kutz
Copyright  2006 by John Wiley & Sons, Inc.

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