Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

same technique provided the data of Tables XI and XVI. Table XI illustrates the important point that all glacial acetic acid is not necessarily the same. This fact is particularly noticeable when comparing two different acids by means of the “hot wall” test. Also note that again a small amount of water in the acid is most helpful in reducing attack on the stainless steels. The water is most effective in this respect regardless of the mode of testing, and field work verifies this inhibitory effect. The effect of adding sulfuric or formic acid to the acetic acid is shown in Table XVI. Notice the accelerating effect of only a small amount of formic acid added to the acetic. Such an addition would produce no increase in corrosion of Type 316 stainless steel in an immersion test conducted at 110 ºC (230 ºF). The effect of adding the even more aggressive, higher boiling sulfuric acid, such as used in an esterification reaction, may be catastrophic as can be observed from the data. 5. Effect of Microstructure The austenitic stainless steels are subject to specific types of attack when exposed to hot organic acids in the same manner as that observed in the mineral acids. Adverse mill treatments, fabrication heating cycles, post-fabrica- tion heat treatment and welding can produce changes in the alloy structure which greatly reduce the corrosion resistance in hot acetic acid. Chromium depletion associated with carbide precipita- tion along the grain boundaries (sensitization) on heating an unstabilized, regular carbon (0.08 C max) stainless steel within the range of 425-760 ºC (800-1400 ºF) gives rise to intergranular attack when the alloy is exposed to hot, concentrated acetic acid. Severe intergranular attack can result in the phenomenon known as “sugaring” or “grain dropping.” The attacked, heat-affected surfaces are left in a very rough condition with a bright, (sugary) faceted surface. If the alloy is sensitized throughout its thickness, such attack may proceed until the entire thick- ness of the metal is penetrated.

Persons evaluating the possible effects of sensitization of an alloy in a specific environment should be aware that a comparison of weight loss measurements between sensi- tized and annealed specimens of the metal are not always an adequate procedure after organic acid exposures. Little difference in weight loss may be noted between the two. In fact, many data indicate that the mass of the austenite grain in a sensitized metal becomes cathodic to the grain boundary which results in a tower overall loss in weight than for the annealed structure (Table XVII is typical). Unless obvious “sugaring” or the dropping of grains from the metal has occurred, the welded or sensitized corrosion test specimen should be evaluated by bending to open and expose the attack, by “ringing” to determine if the metal has lost the characteristic metallic tone, by conducting magnetic permeability tests, or preferably by a metal- lographic examination of a cross section of the metal to observe the type and extent of any selective attack on the structure. Susceptibility of the austenitic stainless steels to this type of attack may be avoided by utilizing a low carbon grade (.03 C max) or restricting the use of regular carbon grades (.08 C max) to the annealed condition, without any subsequent heating into the sensitizing temperature range. With low carbon grades, there is little likelihood of sensitization developing in the alloy during welding or heat treatments. A stabilized counterpart to Type 316 stainless steel known as Type 318 stainless steel is now obsolete because present melting technology can readily attain low carbon levels on a routine basis. The exposure of the chromium-nickel-molybdenum stainless steels after various thermal treatments to a process stream containing acetic acid has been reported by the Welding Research Council. 11 (Table VI.) The corrosion rates obtained were high for such an exposure for reasons not detailed in the stream analysis. Also, the higher corrosion rates exhibited by the Type 316 stainless steel are in conflict with the usual data obtained when comparing the alloy with the Type 316L alloy. However, the data are emphatic in pointing out the effect of adverse heat treatments on susceptible materials. Note particularly the adverse effect of solution annealing followed by a sensitization treatment on the columbium-stabilized Type 318 alloy. This type of treatment can occur during multi- ple-pass welding and may result in “knife-line attack” on stabilized alloys. Although carbide precipitation is the best known and most common cause of intergranular attack on the stain- less steels, certain other metallurgical phenomena must be recognized as presenting potential problems as a result of fabrication procedures. The formation of sigma phase or chi phase in the alloy can be as devastating as carbide precipitation under certain conditions of acetic acid ex- posure. Welding alloys such as Types 316L and 317L stainless steels presents no problems when using solid construction. However, as the process pressure increases and the use of clad construction is indicated to be economically desirable, problems can be encountered if adequate precautions in the fabrication of the vessel are

TABLE XVII

Corrosion in Acetic Acid Vaporizer

Field Test: 312 hr, 140 C (284 F) mass temperature. Chlorides present

Corrosion Rate

Alloy

mm/y

mpy

Type 316 stainless steel, annealed sensitized Type 304 stainless steel CARPENTER alloy 20Cb-3 INCONEL alloy 600 Titanium HASTELLOY alloy C-276

8.13 6.86 33.02 min*

320 270 1300 min*

6.35 6.60

250 260

.08** .08

3** 3

*Dissolved **Pitting

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