CORROSION
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A
Workbook Feature
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m Type 3 1 6 stainless steel pyridine liquor line shows severe corrosive attack which produced grooves or pits on the inside of the pipe
separate glass vessels that contained pyridine liquor, and one of the ves sels was slowly heated. T h e solu tions were connected by a salt bridge, and potential measurements were made with a vacuum-tube voltmeter. T h e results of these measurements indicated that a heated portion of the pipe could be as much as 470 mv. anodic (less noble) to a room-tem perature portion of the pipe, which acted as the cathodic (more noble) member of the galvanic cell. This means that under process conditions, if steam were able to leak into the steam jacket, the portion of the Type 316 pipe that was in the jacket and in contact with pyridine liquor would corrode preferentially to protect the portion of the pipe below the steam jacket. Of course, if enough steam leaked into the jacket, the tempera ture of the acid would raise sub stantially and the acid would become increasingly corrosive. Because process conditions were
such that the use of steam to maintain the temperature of the recirculated pyridine liquor at 50°C. could be eliminated, steam was no longer fed into the steam jackets at any time. T h e failed section of line was re placed about 2 years ago, and no at tack has since occurred on the inside of any of the lines. T h e corrosion resistance and the potential behavior of the stainless steels in sulfuric acid have been ex tensively studied by a number of in vestigators. These studies, as well as practical experience, indicate that the chromium-nickel austenitic stain less steels are highly resistant to dilute sulfuric acid if the proper conditions of temperature, acid concentration, and degree of aeration are main tained. T h e corrosion resistance of these stainless steels depends on the maintenance of a protective or pas sive film on the surface of the metal. Air, or other oxidizing agents in sul furic acid, will extend the range of
Pitting in heat-affected base metal adjacent to weld shows evidence of grain boundary carbide ( X 5 0 0 ) 86 Λ
INDUSTRIAL AND ENGINEERING CHEMISTRY
usefulness of the austenitic stainless steels and help to ensure their corro sion resistance. However, the design of the equipment should be such that the oxidizing agent has free access to the metal surface. Crevices, stagnant areas, and the accumulation of de posits should be avoided. As might be expected, the alloy content of the austenitic stainless steels influences their performance in sul furic acid solutions, with the grades having higher alloy content being more resistant. T h e presence of molybdenum in the alloys is defi nitely beneficial. A factor that may detrimentally affect the behavior of the austenitic stainless steels in sulfuric acid is the presence of precipitated carbides at the grain boundaries of the metal. If these carbides are present, the metal may corrode intergranularly. T h e precipitation of carbides occurs when, as a result of welding or improper heat treatment, the metal is heated in the temperature range of 900° to 1400° F. T h e low-carbon grades (0.03 max. C) or the stabilized grades, AISI Types 321 and 347, are used to avoid intergranular attack. T h e potential behavior of the aus tenitic stainless steels in sulfuric acid is such that these steels m a y exist in an active (anodic) state or in a pas sive (cathodic) state, depending upon the factors previously discussed. Maintenance of a passive state is nec essary for corrosion resistance. How ever, even if the steel does become ac tive, it can often be returned to a pas sive state by lowering the sulfuric acid concentration or temperature. T h e best method of determining the most suitable stainless steel to use is to conduct corrosion tests in actual plant equipment. If only laboratory tests can be made, careful interpreta tion of the results is required. Cor rosion test facilities should then be built into new equipment to follow the corrosivencss of the solution and to determine when the process is out of control.
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by W. L. Mathay, Applied U. S. Steel Corp.
Research
Laboratory
1m
CORROSION A
W O R K B O O K
F E A T U R E
Sulfuric Acid Corrosion of Stainless Steels To determine the most suitable stainless steel, conduct corrosion tests in actual plant equipment
S T A I N L E S S STEELS are extensively used in chemical industry, and their Versatility has greatly aided the in dustry's advancement. The chro mium-nickel austenitic stainless steels have proved particularly useful be cause they combine excellent corro sion resistance with desirable me chanical properties. However, to ob tain the greatest economic advantage from the use of stainless steels, the environment and the design of the equipment in which these steels are used must be properly selected. For example, in many coal-chemi cal plants, AISI Type 316 stainless steel (nominally 17% chromium, 12% nickel, and 2.5% molybdenum) is often used as a construction ma terial for vessels and piping that are in contact with solutions which have sulfuric acid contents of 10% or less. I n one such plant that has 18 recycle lines of Type 316 stainless which are exposed to acidic pyridine liquor, one of the lines failed after about 2 years' service. None of the other lines showed evidence of corrosive attack. The liquor contains pyridine, its homologs, and about 10% sulfuric acid, and it is probably aerated to some extent. A portion of the Type 316 recycle lines was steam-jacketed to maintain the temperature of the liquor at about 50° C. when the socalled "pyridine bases" were being recovered. When the bases were not being recovered, the steam was shut off and the pyridine liquor remained stagnant in the recycle line at a level equivalent to the liquor level in the pyridine-storage · tank. Under the latter conditions, the level of stagnant liquor would be inside the steamjacketed portion of the pipe (see dia gram) .
Examination of the Type 316 pipe which failed revealed that the failure had occurred in that portion of the pipe which was steam-jacketed and in contact with the stagnant pyridine liquor. Below this area the pipe was relatively unattacked. The photo graph (top, page 80 A) shows the very severe corrosive attack on the interior of the pipe. The photomicrograph (bottom,-page 80 A) shows attack in the heat-affected base metal adjacent to a weld in the pipe. Some intergranular carbide precipitation had occurred in the heat-affected areas, and these areas appear to have been preferentially attacked. Although Type 316 would not be expected to be completely resistant
to air-free 10% sulfuric acid at 50° C , the very severe corrosive attack which took place must be attributed to some other factor such as sulfuric acid at concentrations and/or temperatures in excess of those normally en countered in the equipment. Elec trochemical measurements were made in the laboratory to ascertain whether differences in potential could exist between sections of the failed pipe exposed to the pyridine liquor at different temperatures. Two sec tions of the failed pipe were cleaned and masked with polyethylene tape so that on each section, only a 2square-inch area of the uncorroded exterior of the pipe was exposed. The sections were then placed in
θ RECYCLE LINE AISI T Y P E 316 SS
-STEAM PYRIDINE
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This stainless steel p y r i d i n e t a n k a n d recycle line h a d cor rosion f a i l u r e only where the liquor remained stagnant in the steam-jacketed portion o f the line b e t w e e n operations
I/EC
O R K B O O K FEA.TrCrR.E3S
85 A
