EFFECT OF ALLOYING ELEMENTS -2

When one speaks about the effect of this or that pure chemical on an alloy, one must emphasize that real industrial environments are complex mixtures of chemicals. These mixtures may behave in surprising ways, quite unlike what one might expect from the behavior of alloys in pure, laboratory-controlled environments. Corrosion rates depend not only upon the concentrations of various chemicals but also on the temperature. The temperature of liquid inside a vessel is one point that can be measured, but the temperature at the surface

of submerged heating coils in that vessel is another and higher value. Likewise the concentration of, say, an acid in the vessel is not the same as the concentration at the point where that acid is introduced to the mixture.

The most commonly used corrosion-resistant alloys are the stainless steels 304 (18% Cr–8% Ni, commonly known as 18-8 stainless steel) and 316 (17% Cr–11% Ni–2% Mo).

The more corrosion resistant nickel alloys, such as C-276, have much higher levels of nickel, 57%, and molybdenum, 15.5%. Commercially pure nickel and nickel–copper alloys are used for special environments.

Oxidizing and reducing environments are defined chemically with respect to whether hydrogen is oxidized or reduced under the environment in question. In an oxidizing environment, hydrogen will only be present chemically combined with some other element, for example with oxygen to form H2O. In a reducing environment that H will be reduced to hydrogen gas, H2. Common oxidizing chemicals are nitric acid, HNO3, and certain salts such as ferric chloride, FeCl3, and cupric chloride, CuCl2. The ferric and cupric ions are at relatively high valences, 3 and 2, respectively, and readily accept electrons from, or oxidize, other materials

to get their own valences reduced to a more stable level. Sulfuric acid, H2SO4, is normally a reducing acid.

At high concentrations, above about 95%, sulfuric acid changes its character and becomes an oxidizing acid. Of course, dissolved oxygen contributes to the oxidizing character of an environment. To some extent so does dissolved elemental sulfur.

To resist oxidizing conditions, an alloy must contain some amount of chromium. In an oxidizing acid, simple materials such as 304 (18% Cr–8% Ni) or 310 (25% Cr–20% Ni) are often used. An unusually high level of chromium, 33%, is present in a newly developed alloy, UNS R20033, meant to resist very oxidizing acids. In any of these alloys the nickel content is necessary to make a stable austenitic alloy, but it does not contribute specifically to oxidizing acid resistance. Small additions of molybdenum or copper may be tolerated in

these alloys to enhance resistance to chlorides or sulfuric acid. But neither Mo nor Cu is helpful in resisting strongly oxidizing chemicals.

A common and severe test for resistance to oxidizing acids is boiling 65% nitric acid.

The test is run for five periods of 48 h each, specimens being weighed after each test period,

and the results averaged. This test is a good measure of resistance to intergranular corrosion

in a sensitized alloy as well as to general corrosion in nitric acid. Test results2 show 2205

at 0.13–0.20 mm/yr, which is good; 304 at 0.23 mm/ yr; and RA333 , which has been

stabilize annealed at 1700 F, at 0.29 mm/yr. In the case of RA333 it is the high chromium

that helps, in spite of the 3% Mo. Other molybdenum-bearing grades do not fare so well, at

316L (2% Mo) at 0.87 mm/yr after only 24 h, AL-6XN (6.3% Mo) at 0.74 mm/yr, 625

(9% Mo) at 0.76 mm/yr, and C-276 (15.5% Mo) at 0.74 mm/yr. These results do not mean

that one cannot successfully use a higher Mo alloy in the presence of any nitric acid at all.

They do indicate that high molybdenum alloys may not behave at all well in hot, concentrated

oxidizing industrial environments.

One cannot readily find boiling 65% nitric acid (ASTM A 262 C) data for the 66% Ni–

31% Cu alloy 400 (Monel 400) or for the assorted B alloys—B, B-2, B-3, or B-4. Their

corrosion rates in nitric acid are simply too high for the test to have any practical value.

Alloys 400, B, and B-2 have no deliberate chromium addition, B-3 and B-4 only about 1.3%

Cr. These grades may have excellent resistance to various reducing environments, but because

there is essentially no chromium present, they will literally dissolve in nitric acid. Likewise,

they are attacked by ferric, cupric and chlorate ions and even dissolved oxygen in HCl.

The common ‘‘reducing’’ acids are sulfuric under about 95%, phosphoric (H3PO4), and

hydrochloric. Of these by far the most corrosive is HCl, phosphoric being the less troublesome.

Because reducing industrial environments often do contain some oxidizing salts or

oxygen from the air, most alloys used to withstand reducing chemical environments will

contain chromium, at least 15%. The alloy additions used to resist the reducing components

of the environment are nickel (Ni), molybdenum (Mo), and copper (Cu).

In sulfuric acid some amount of copper is usually used, such as in 20Cb-3 stainless

steel, 904L, or 825. Even copper salts in the acid will reduce corrosion attack on stainless

steel. 20Cb-3 uses carefully balanced proportions of Cu and Mo to resist sulfuric acid corrosion.

READ MORE.......

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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