The corrosion of materials by organic acids is complicated by the virtually unlimited number of possible compounds. The corrosion of metals by organic acid is often confounded by trace impurities such as oxygen and metallic salts. This paper concentrates on corrosion by acetic, formic and propionic acids and gives some information on longer chain organic acids.
A key group of industrial chemicals is organic acids. They are often used in their pure form and they are used as an intermediate in a wide variety chemical reactions to make products ranging from polyester clothing to amino acids used in vitamins. Acetic acid is synthetically produced in the largest volume of all of the carboxylic or organic acids and is best known by the general public as the weak aqueous solution, 'Vinegar'. The simple, straight chain aliphatic acids are discussed in this paper. They are often called "fatty acids" because those containing an even number of carbon atoms (four or greater) exist in a combined form with glycerol as fats and oils. l
Since there are almost an unlimited number of organic acids possible, the subject is complicated. Often the acids are not handled as pure products but as mixtures with inorganic acids, salts, a wide variety of organic solvents, and in mixtures with other organic acids. They have odors that vary from sharp like formic and acetic acid, to rancid like butyric acid (butter) or even worse, smell like sweaty goats - caproic acid.
CORROSION CHARACTERISTICS
Organic acids are weak acids when compared to the common inorganic acids like HC1 or H2SO4 but still hydrolyze well enough to act as true acids toward most metals. Aliphatic organic acids are usually considered to be slightly reducing. They are often handled in copper which does not directly displace hydrogen from acids. The 400 series stainless steels exhibit borderline passivity and are thus are seldom selected whereas the 300 series stainless steels are today's work-horse. The 300 grade of stainless steels require oxidizing conditions to maintain their passivity, especially at high temperatures. The reversal of corrosion resistance as the environment changes from oxidizing to reducing characteristics make contaminants extremely important because they tend to shift the oxidizing capacity of the acid mixture. Aeration (i.e. dissolved oxygen or DO), ferric ions, peracids or peroxides will cause rapid attack of copper and copper alloys while the presence of chlorides, which are reducing, can have disastrous effects on stainless steels. 2
Corrosion testing in organic acid media can be either difficult or misleading, mainly because of the effect of impurities. Electrochemical measurements are used almost exclusively in the dilute aqueous solutions of organic acids, wherein hydrolysis effects improve conductivity. Electrical conductivity is very low in high concentrations of organic acids and in solutions in non-aqueous solvents like benzene. The addition of sodium or chloride salts is reported to allow electrochemical measurements in these types of solutions. 3 Electrochemical data obtained in strong acetic acid, acetic acid-anhydride and formic acid solutions showed active-passive behavior for stainless steels and nickel containing alloys. This is consistent with field experience. 4
Laboratory data from by immersion tests often show erroneous results unless the atmosphere is carefully controlled. Short tests of metals, especially those that exhibit active-passive behavior, can be misleading because the metal may remain passive during the initial exposure only to corrode in an active state at a rapid rate after longer exposure. Without atmospheric control, the solution will
