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IT SEEMS OBVIOUS that before anyone can undertake anything but trial and error or empirical methods of solving corrosion problems, he must have basic information as to the nature of corrosion and the mechanics of corrosion processes. With this knowledge he can make a faster and more accurate diagnosis or analysis of any problem that arises and will be in a much better position to reason from one experience to another. It will also permit him to appraise the information presented to him, plan research to uncover new information, and interpret and apply results of investigations when they have been completed.
No one can expect to choose the proper material to overcome a corrosion problem unless he knows what materials are available and something about their corrosion resistant qualities-advantages and limitations. He does not need to keep all of this in his head, but he should know where to find information when he needs it. This may be located in books or other publications in his library or files or he may know where the information is most likely to be secured elsewhere by telephone, telegram, mail, or direct consultation.
In dealing with a corrosion problem, the nature of the corrosive medium is obviously as important as the properties of the material itself. It is therefore necessary to know a good deal about the corrosive characteristics of the chemical or chemicals involved and how these are affected by such factors as concentration, temperature, velocity, aeration, or the presence of oxidizing or reducing substances or special contaminants. This will obviously influence the choice of material or indicate what changes in important environmental factors may be expected to have a favorable effect in reducing corrosion of existing equipment.
Corrosion resistance alone is not sufficient to qualify a material for a particular service. It also must be strong enough to sustain the mechanical stresses that may be encountered. Thermal conductivity may be important where high rates of heat transfer are required, or the thermal expansion should be such as to permit association with some other material without intolerable distortion or the development of excessive thermal stresses.
Regardless of how attractive a material may be from any other point of view, it is of no use for a particular purpose if it cannot be secured in the required form. Filter cloth cannot be woven from an alloy available only as castings. Several materials may possess the corrosion resistant and mechanical properties required for a job, but many of them may be too expensive to be considered. For example, silver might be somewhat better than nickel for tubes in an evaporator to concentrate caustic soda to 50%, but it would not be enough to justify the extra cost involved; steel might be a better choice economically overall for handling dilute caustic under less stringent conditions. Steel is the economically practical material for tank cars for shipping concentrated sulfuric acid, although there are many strong complex alloys that are corroded by sulfuric acid much less than steel under the conditions that may be encountered in tank cars.
A corrosion engineer must not only be familiar with all of the common practices in the working and fabrication of metals, but also know which materials are amenable to fabrication by the procedure that is to be used. It is useless to choose a metal that cannot be welded for a vessel that cannot be fabricated in any other way. It is equallY as useless to specify an alloy that is difficult to machine for a complicated part that must be finished to close tolerances and be capable of being turned out in quantity on automatic machines at a reasonable cost.
It is often necessary to choose materials in such a way as to avoid any chance that significant amounts of particular metal compounds may be picked up by the product that would impair important properties such as clarity, color, flavor, strength, or keeping quality, or otherwise restrict the opportunity for its use and sale in some important market.
Existing information must frequently be supplemented by developing new data needed to solve a particular corrosion problem. It is necessary to be familiar with the several types of corrosion tests that may be made and the advantages and disadvantages of each so as to choose the best approach. The techniques of carrying out the required tests should be understood so that the tests may be conducted properly. The limitations of the tests must be appreciated so that the results of tests may be appraised properly and applied intelligently.
The work of a corrosion engineer will bring him into frequent contact with responsible people in many other branches of his organization:
No comment is required beyond the reminder that all of the other attributes will be greatly increased in value if they are always tempered with common sense.
F.L. LaQue, What can management expect from a corrosion engineer?, Materials performance, August 1985, pages 82-84. Copyright NACE International, reproduced with permission.