Nature and Extent of the Corrosion Problem
Sensational figures have been published with regard to the annual cost of corrosion and of protection against corrosion. One estimate places this loss at over 6 billion dollars. An accurate estimate is, of course, impossible, but it is not difficult to perceive that the cost is enormous when we consider that corrosion' occurs, with varying degrees of severity, in practically all cases where metals and alloys are used. Everyone is familiar with the rapid ,rate at which iron and steel corrode or rust when exposed to atmosphere and rain. (reference)
Some of the fluids which cause corrosion difficulties serious enough to warrant actual investigation are the following: fresh, distilled, salt, and mine waters; rural, urban, and industrial atmospheres; steam and other gases such as chlorine, ammonia, 'oxygen, carbon disulfide, sulfur dioxide, and fuel. gases; mineral acids such as nitric, sulfuric, and hydrochloric; organic acids such- as acetic, formic, and-citric; alkalis such as caustic and ammonium hydroxide; soils; solvents such as alcohols and dry cleaning materials; vegetable and petroleum oils and a variety of food products. Two of the. most common and most plentiful materials known - namely, air and water - also cause corrosion, and considerable effort and money have been spent to minimize their destructive effects.
Aside from the cost in dollars, corrosion is a serious problem because it directly and definitely contributes to the depletion of our natural resources. For example, steel is made from iron ore and our reserves of iron ore are diminishing. In addition, approximately 4 tons of coal are required to produce I ton of steel. Our copper reserves are dwindling, and copper is one of the principal elements used in the production of corrosion-resistant alloys.
The war years have placed further accent on corrosion. We often read about the millions of dollars worth of equipment which is rusting and deteriorating, particularly in tropical, and marine atmospheres. We know, too, that failure of parts or failure of equipment to function properly because of corrosion -often seriously handicapped our military' forces. These facts account for the large amount of effort. expended during the war, and now in the postwar period, on studying 'and combating corrosion. The increased effort and interest are reflected in the corrosion "forums" that are being held throughout the country and in the large number of technical society sessions and papers devoted to the subject.
Corrosion is truly a major industry but, unfortunately, one which is "in reverse." Its waste obliges all concerned to minimize or eliminate it in so far as possible.
In addition, chemists and chemical engineers have other vital interests in the corrosion' problem. Some of their reasons for concern are described in the following paragraphs.
Higher Temperatures and Pressure
The trend, conspicuous during recent years, in the chemical industry toward higher temperatures and pressures have made possible new processes or improvements in old processes - for example, better yields, greater .speed, or lower cost of production. Higher temperatures and pressures usually involve more severe corrosion conditions. Many of the present day operations would not have been possible or economical without the use of corrosion-resistant materials. The annual tonnage of stainless steel purchased by the chemical industry today is many times greater than that purchased ten years ago. A powder plant of World War II would be almost unrecognizable, from the standpoint of materials of construction, to an operator of a similar plant in World War I.
Reduction In Maintenance Costs
Substantial savings can be obtained in most types of chemical plants through the use of corrosion-resistant materials of construction. One example is classic in this respect. A plant effected an annual saving of more than 10,000 dollars merely by changing the bolt material on some equipment from one alloy to another more resistant to the conditions involved. The cost of this change was negligible. In another case of waste-acid recovery plant operated in the red for several months until a serious corrosion problem was solved. This plant was built to take care of an important waste disposal problem. Maintenance costs are now scrutinized because the labor picture accents the necessity of low cost operation.
Contamination of Product
In many cases the market value of a chemical plant product - for example, titanium dioxide pigments is directly related to its purity and quality. Freedom from contamination is also a vital factor in the manufacture of transparent plastics, food products, and drugs. In some instances contamination causes adverse catalytic effects which are noticeable - for example, in the manufacture and transport of hydrogen peroxide.
Life of the equipment is not generally an important factor in cases where contamination or degradation of product are concerned. Ordinary steel generally lasts many years, but more expensive material is used because the presence of iron rust is undesirable from the product standpoint.
The history of sodium hydroxide presents an interesting example of improvement of a chemical plant product through the use of corrosion-resistant materials. The variety of colors in the first commercial caustic produced was due principally to the iron pick-up from the cast iron pots used.
Caustic was regarded as an extremely corrosive material, and its manufacture and use were not greatly encouraged. During the late twenties the rayon industry demanded a high purity caustic because any impurities were extremely detrimental to its product. During recent years white or rayon-grads caustic has been accepted as a normal commercial item. Its impurities are low and usually expressed as parts per million. Nickel and nickel alloy equipment play an important part in its production and handling. Natural and synthetic rubber linings and paints also prevent its contamination by steel equipment.
High purity caustic has played in important part in the development of high strength rayon for tire cord during the war, for photographic film, and for high grade soaps.
Far too often plants are shut down or portions of the process stopped because of unexpected corrosion failures. Perhaps nothing is more exasperating to management or industrial plant staffs, particularly during periods of high demand for the product, when a direct loss in revenue is involved. Sometimes these shutdowns are caused by corrosion involving no change in process conditions; but often they are caused by changes in operating procedures regarded as incapable of increasing the severity of the corrosive conditions. It is surprising how often some minor change in process or the addition of a new ingredient changes completely the corrosion picture. The production of a chemical compound vital to aircraft performance during a critical period of the last war is an example. To increase its production, the temperature of the cooling medium in a heat exchanger system was lowered and the time required per batch decreased. Lowering the temperature of the cooling medium resulted, however, in more severe thermal gradients across the metal wall. They, in turn, induced higher stresses in the metal. Stress-corrosion cracking of the vessels occurred quickly, and finally a plant was shut down with production delayed for some time.
Sudden and rapid corrosion of stainless steel equipment caused another example of plant shutdown. The cause of the accelerated corrosion and crippling operation had to be determined and corrected before operation could be resumed, because expensive and hard-to-obtain stainless centrifugals were involved. The accelerated attack was finally traced to faulty valves which allowed leakage of hydrogen sulfide into the system.
Particularly during recent years, the products or intermediates handled in a chemical plant are often expensive. Substantial production losses of these materials to the sewer because of corrosion failures are, of course, to be avoided.
Last but not least is the consideration of this factor. The handling of chemicals at high temperatures and pressures, explosive materials, and acids such as hydrofluoric and concentrated sulfuric demand materials of construction which minimize corrosion failures if severe injury or loss of life are to be avoided. Corroding equipment is known to have caused fairly harmless compounds to become explosive. Economizing on materials of construction is generally not desirable if safety is risked.
The chemist and the chemical engineer should be familiar with corrosion and the corrosion problem. Each should be able to appreciate the problems involved, and should know where to obtain the necessary information or how to go about obtaining it, if he has to do the job himself. Many of the larger 'companies have materials engineers or corrosion engineers trained and experienced in this type of work. In many plants, especially the smaller ones, however, the chemist or the chemical engineer has to handle the work himself. So-called details are often of major importance. For example, the correct stainless steel may be specified for a 'given application but the results are often poor if the material is not properly hent-treated.
Corrosion may be defined as the destruction or deterioration of metals and alloys by chemical reaction with their environments. The definition can be amplified by including electrochemical reaction if we wish to differentiate between the two general classes of corrosion - namely, chemical and electrochemical. Chemical corrosion involves direct chemical attack, and electrochemical corrosion concerns destruction by electrolytes. These two classes could be more simply described as dry and wet corrosion.
This definition holds for the corrosion of metals and alloys. In a broader sense of the word, however, other materials also corrode. The slag in a steel-making furnace fluxes the brick lining, acids and other chemicals attack rubber and plastics, and sunlight deteriorates paints. These are all examples of corrosion. Corrosion can therefore be defined simply as the deterioration of materials by chemical attack.