Corrosion—Process Units

What are the two most powerful corrosive agents in a process plant Not acids or sulfur, or caustic or salt. Neither cyanides nor carbonates nor chlorides. The most aggressive corrosive agents are air and water. Nitrogen is inert. It is oxygen in an aqueous environment that is the main cause of many corrosion problems. 

Rusty carbon steel pipe is the commonly encountered form of corrosion due to air and water. Stainless steel appears to be immune to such corrosive attack, but this is not quite true. Stainless steel contains a substantial concentration of chrome and nickel. Common 304 stainless steel has 18 percent chrome and 8 percent nickel. 

The chrome especially is subject to oxidation when exposed to wet air. But the metallic oxides so formed adhere tightly to the surface of the metal. This rugged metallic oxide layer acts as a barrier to prevent further destructive oxidation of the base metal. The oxide layer is so thin that it is smaller than the wavelength of visible light. Thus, it is quite invisible to the human eye. Stainless steel piping appears shiny after years of service, even though its surface has been oxidized after just a few minutes of service. 

Anyone who has ever painted a wrought iron fence to prevent rust appreciates the concept of a protective barrier. Aluminum and titanium are protected from corrosion due to the rapid formation of a surface metallic oxide layer, which tightly adheres to the metal surface. 

The problem with ordinary carbon steel is that the oxide formed does not stick very tightly to the metal surface. The essence of corrosion control then is to manipulate process parameters so that a protective barrier of products of corrosion does adhere to the surface of carbon steel vessels and piping. 

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As already noted (Chapter 3), petroleum oil often contains water, inorganic salts, snspended solids, and water-soluble trace metals. As a first step in the refining process, to reduce corrosion, plugging, and fouling of equipment and to prevent poisoning the catalysts in processing units, these contaminants must be removed by desalting (dehydration). 

It is certainly my experience that the most common and catastrophic accidents in process units are related to corrosion-type failures. I cannot actually bring to mind any equipment vessels which were overpressured and failed because a relief valve did not open. 

On older process units, you may still encounter piping with sentry holes. Let s say I have a Vfc-in thick pipe. The corrosion allowance for the pipe is Vi in. A number of small holes are drilled into the pipe, to a depth of in. When we start leaking at these small holes, this means that the pipe has corroded to its discard thickness in the area of the sentry hole. Incidentally, you can stop the resulting leak, at least in carbon steel water lines, with a brass wood screw and a screwdriver. I have also done this on hydrocarbon lines under an 80 psig pressure, but perhaps that is not too smart. 

Shortening units are constructed of carbon steel. The water phase in margarine is corrosive, and sanitation procedures require that all of the equipment used to manufacture it must be chemically cleaned. Margarine processing units contain chromium-plated commercially pure nickel heat transfer tubes and stainless steel for all product-contacted metal surfaces. 

The index works out at 21 classified as Light. Ammonia would not normally be considered a dangerously flammable material the danger of an internal explosion in the reactor is the main process hazard. The toxicity of ammonia and the corrosiveness of nitric acid would also need to be considered in a full hazard evaluation. The process unit risk analysis would be completed when the site for the plant had been determined. 

The data set of metal corrosion provides a valuable insight as to which parameters dominate the corrosion process. The general meteorological features are well established in North America (although micrometeorological factors may dominate at any single exposure site). The pollutant related questions may be simply stated (1) what are the magnitudes of marine and industrial corrosion rates compared to the rural values (2) what are the relative rates in the rural eastern United States compared to the western rural values (3) what are the broad trends in corrosion rate over the years ... 

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