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Galvanized steel stress corrosion cracking mechanism series#

The adoption of these recommendations was very effective in practically eliminating SCC in carbon-steel vessels used in the agricultural industry. The report also remarked that, inasmuch as each of these recommendations has limitations when considered individually, all three should be followed to avoid SCC.

Ammonia should contain at least 0.2 percent water to inhibit SCC.

Extreme care should be used to eliminate air from the ammonia systems new vessels must be thoroughly purged to eliminate air contamination.

Pressure vessels should be either fully stress relieved or fabricated with heads that are hot-formed or stress relieved.

As the result of the studies the research committee recommended that: (Although at that time the normal carbon dioxide content of air was believed to be essential to the cracking process, later studies modified that concept.) It was also shown repeatedly that at least 0.1 percent of water in the liquefied ammonia inhibits SCC of constructional steels used in the fabrication of the pressure vessels for agricultural service. Of the many contaminants that could have been present in the ammonia, air was identified as the specific agent which caused SCC. The probability of SCC increased with increased stress level as well as with increased yield strength of the steel. By evaluating the storage and handling facilities and practices for liquefied ammonia and by conducting controlled tests, it was determined that stress-corrosion cracking (SCC) was the most likely cause of the failures. Thereupon a research committee was formed by the Agricultural Ammonia Institute to determine the causes of equipment failure and to recommend palliative measures.

A survey indicated that about 3 percent of the pressure vessels failed within 3 years of service life **.

Although liquefied ammonia had been used without significant difficulties for several decades in the refrigeration, chemical and steel heat-treating industries, inexplicable ruptures of ammonia containers in the agricultural service occurred soon after the introduction of ammonia into this service. Because excellent results were achieved, very soon a large network of ammonia distribution and application facilities developed, which used many pressure vessels constructed primarily of carbon steels. Soon after World War II the Mississippi State College proposed a method of injecting liquefied ammonia directly into the soil as a chief source of nitrogen fertilization (ammonia contains about 82 percent nitrogen). Control measures are discussed including control of air contamination, inhibition by water addition, postweld heat treatments, influence of the strength of the steels used and the applied stress, and periodic inspection techniques. Stress-corrosion cracking of steel in liquefied ammonia is traced from its early recognition in the 1950s, from experiences in agricultural and industrial use from NACE (National Association of Corrosion Engineers) recommended DOT (Department of Transportation) regulations to prevent damage, and from laboratory studies to explain the factors involved. The review is aimed toward a summary of the conclusions from various sources specific details are available in the referenced reports. From the many reports reviewed, an effort was made to concentrate on those that would lead to a better understanding of the problem facing the industry today and suggest a solution to the problem. This recapitulation of experiences with stress-corrosion cracking of steel in liquefied ammonia summarizes the subject over the last three decades. The following article is a follow-up to "Industry Study Indicates Stress Corrosion Cracking in Anhydrous Ammonia Storage Vessels," which appeared in the October 1988 BULLETIN.

Galvanized steel stress corrosion cracking mechanism series#

Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. Self-employed consulting corrosion engineer retired from U.S. Stress Corrosion Cracking of Steel in Liquefied Ammonia Service - A Recapitulation