May 1949
Corrosion Environment and corrosion testing of small pieces of equipment in the pilot plant are described by Mars G. Fontana
P
REPARATION and identification of specimens, .their location and support, and time and temperature factors in connection with pilot plant corrosion testing were discussed in March and April. This column concludes this series of three articles. In general, these discussions also apply to actual plant or field tests. A factor which must be considered most carefully in corrosion testing is the environment or the corroding medium. Needless to say, unreliable and often worthless results can be obtained if the tests are made in an environment that is essentially different from contemplated actual conditions. This is one of the important reasons why pilot plant testing is extremely desirable and valu able. In most cases, commercial materials and not e.p. reagents are handled in the pilot plant. It is a well known fact that minor, and so-called unknown, impurities often change completely the corrosion picture or the life of equipment. An excellent example concerns hydrochloric and muriatic acids. This acid is quite corrosive and small amounts of impuri ties often increase the severity of corrosion, c.p. hydrochloric acid is somewhat different from ordinary commercial muriatic acid. The problem is further complicated by the increasing use of recovered or by-product hydrochloric acids in the chemical and petroleum industries. In many instances, these acids contain appreciable amounts of iron which means that they often contain ferric chloride. This iron is picked up in the plant operations be cause of corrosion in the process. Ferric chloride is one of the
most destructive pitting agents known. Cupric chloride is another bad actor in this respect. The corrosion picture could change also, in the other direction, how ever, in that sometimes these recovered acids contain organic matter which may act as an inhibitor and actually decrease the aggressiveness of the acid. In some cases, the compositions of the fluids handled are confidential in nature and outside help and vendors of equipment cannot be furnished this information. This situation is remedied iti the pilot plant in that the actual fluids are handled. The pilot plant tests also take care of other factors such as changes in the chemical and physical nature of the environment during operation; intermittent changes in environment, which are often encountered in batch operation; relations between the vol ume of liquid handled and surface area of the equipment; vibra tion; velocity effects ; catalysis effects; pressures; and tempera ture fluctuations. This latter item is generally overlooked, but in several cases of severe and disastrous stress-corrosion failure, in vestigated by the writer, the primary cause of failure was the very high stresses induced in the metal because of severe thermal gradients. Stresses of the order of 50,000 pounds per square inch were involved. This factor is so important that the author is planning an investigation, {Continued on page 96 A)
NO A I R LEAKAGE t h r u stuffing b o x into v a c u u m
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TABER
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Engineers
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Corrosion
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to be reported hero, on the effects of thermal gradients on stresscorrosion.
For o WIDE RANGE OF MATERIALS
Actual
ft/niiniÊfui
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Much of the discussion so far concerns exposure of test specimens in the pilot plant. However, the wisdom and desirability of testing actual equipment should bo emphasized strongly. This testing often can be done with relatively little expense. Small valves and pumps, pump impellers, short sections of pipe, heat exchanger tubes, bolts, and other hardware can be installed in the pilot plant and their performance followed. These items usually are small enough to permit fairly accurate weighing, and in this type of test the design factor is automatically incorporated. Design often has an important effect on corrosion performance of equipment. Other factors mentioned in the preceding paragraph are also included. Semiworks chemical engineers and chemists usually are primarily concerned with proving a given process but they are willing to install actual parts or specimens if this matter is brought to their attention. Close cooperation between the operator and the corrosion man are certainly indicated and recommended. In many instances the operators are not well grounded in corrosion and they can obtain much information from the manufacturers and vendors of corrosion resisting materials and equipment.
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Figure 1. Panel Exposure Test in Pilot Plant Fume Stack If previous experience or background does not eliminate all except a few materials for consideration, it would be desirable to conduct laboratory screening tests to narrow the field of possibilities. This procedure simplifies the situation with regard to testing actual parts in the pilot plant. Figure 1 shows a method of testing metal and protected metal panels in a pilot plant stack handling corrosive fumes and gases. Note that the stack itself is made of wood. For economic reasons pilot plant equipment is often made as cheaply as possible because these plants run for relatively short times. This fact again emphasizes the need for corrosion testing in the pilot plant. Pilot plant corrosion testing is very important and should be carried out if possible. The actual plant conditions are usually duplicated in these tests.
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HUH Combined reprints of this series of three columns on pilot plant cor.rosion testing will be available on or about M a y 20. Price, 25 cents. Address Reprint Dept., 115S-16th St., N.W.. Washington 6, D . C.
DRUMS 96 A
