Industrial atmospheric corrosion, steels

With this requirement profile, these steels can also be considered for applications in marine atmosphere or industrial atmospheres. The steels 1.4003 (X2CrNil2) and 1.4512 (X2CrTil2) are not likely to show any corrosion-related cross-sectional reductions, even over periods of several decades, with the exception of highly aggressive conditions. Crevice corrosion can be expected under heavy chloride exposure and in the presence of crevices. Such critical conditions do not result from seawater exposure only, but are also seen inland due to deicing salt, etc. The ferritic chromium steels are not, by the way, formulated with crevice corrosion resistance in mind [100]. 

It is generally conceded that steels containing only very low amounts of copper are particularly susceptible to severe atmospheric corrosion. In one test over a 3%-year period in both a marine and an industrial atmosphere, a steel containing 0.01 percent copper corroded at a rate of 80 //m/y, whereas increasing the copper content by a factor of five reduced the corrosion rate to only 35 //m/y. Further additions of small amounts of nickel and chromium reduced the corrosion rate to 10... 

It is also of interest to note that Wranglen considers that the decrease in the corrosion rate of steel in the atmosphere and the pitting rate in acid and neutral solution brought about by small alloying additions of copper is due to the formation of CU2S, which reduces the activity of the HS and Scions to a very low value so that they do not catalyse anodic dissolution, and a similar mechanism was put forward by Fyfe etal. to explain the corrosion resistance of copper-containing steels when exposed to industrial atmospheres. 

Weather conditions at the time of initial exposure of zinc and steel have a large influence on the protective nature of the initial corrosion products This can still be detected some months after initial exposure. Finally, rust on steel contains a proportion of ferrous sulphate which increases with increase in SO2 pollution of the atmosphere. The effect of this on corrosion rate is so strong that mild steel transferred from an industrial atmosphere to a rural one corrodes for some months as though it was still exposed to the industrial environment. ... 

Sulphur dioxide in the air originates from the combustion of fuel and influences rusting in a number of ways. For example, Russian workers consider that it acts as a cathodic depolariser , which is far more effective than dissolved oxygen in stimulating the corrosion rate. However, it is the series of anodic reactions culminating in the formation of ferrous sulphate that are generally considered to be of particular importance. Sulphur dioxide in the air is oxidised to sulphur trioxide, which reacts with moisture to form sulphuric acid, and this in turn reacts with the steel to form ferrous sulphate. Examination of rust Aims formed in industrial atmospheres have shown that 5% or more of the rust is present in the form of iron sulphates and FeS04 4H2 0 has been identified in shallow pits . 

In atmospheric exposure 18% Ni maraging steel corrodes in a uniform manner , and becomes completely rust covered. Pit depths tend to be more shallow than for the low-alloy high-strength steels. Atmospheric corrosion rates in industrial (Bayonne, New Jersey) and marine (Kute Beach, North Carolina) atmospheres are compared with those for low-alloy steel in Figs. 3.29, 3.30 and 3.31. The corrosion rates drop substantially after the first year or two and in all cases the rates for maraging steel are about half the corrosion rate for HY80 and AISI 4340 steels. 

Fig. 3.29 Corrosion rales of maraging and low alloy steels in an industrial atmosphere at Bayonne, N.J. (after Kenyon, Kirk and van Rooyen, Corrosion, 27, 390 (1971) )...

Corrosion rates in normal industrial atmospheres measured as loss of weight over a period are extremely uniform among the various alloys. Table 4.19, last column, gives the corrosion rates (in g m d" ) for a number of alloys determined at Clifton Junction in recent years. The highest value recorded (0-4 g m d ) is equivalent to a rate of penetration of 0-076 mm/y, which is appreciably less than that of mild steel. 

Industrial atmospheres usually accelerate the corrosion of zinc. When heavy mists and dews occur in these areas, they are contaminated with considerable amounts of acid substances such as sulphur dioxide, and the film of moisture covering the metal can be quite acid and can have a pH as low as 3. Under these conditions the zinc is dissolved but, as the corrosion proceeds, the pH rises, and when it has reached a sufficiently high level basic salts are once more formed and provide further protection for the metal. These are usually the basic carbonate but may sometimes be a basic sulphate. As soon as the pH of the moisture film falls again, owing to the solution of acid gases, the protective film dissolves and renewed attack on the metal occurs. Hudson and Stanners conducted tests at various locations in order to determine the effect of atmospheric pollution on the rate of corrosion of steel and zinc. Their figures for zinc are given in Table 4.34 and clearly show the effect which industrial contamination has on the corrosion rate. 

An interesting development in weldable corrosion-resistant steels is the copper-bearing or weathering steels (Section 3.2) which exhibit enhanced corrosion resistance in industrial atmospheres in the unpainted condition. For optimum corrosion resistance after welding, the filler employed should be suitably alloyed to give a deposit of composition similar to that of the steel plate... 

Tests by Clark for the Corrosion Sub-committee of the American Welding Society were carried out at severe marine and industrial sites. After four years, the greatest protection to steel was given by sprayed aluminium coatings combined with aluminium vinyl paint in the following environments id) sea air, ib) sea-water immersion, (c) alternate sea-water immersion and exposure to air (tidal conditions) and id) industrial atmospheres contaminated with sulphur compounds. 

Paint for structural steelwork is required mainly to prevent corrosion in the presence of moisture. In an industrial atmosphere this moisture may carry acids and in a marine atmosphere this moisture may carry chlorides. Paint is therefore required to prevent contact between steel and corrosive electrolytes, and to stifle corrosion, should it arise as a result of mechanical damage or breakdown of the coating through age and exposure. 

Tin—Nickel, Alloy deposits having 65% tin have been commercially plated since about 1951 (135). The 65% tin alloy exhibits good resistance to chemical attack, staining, and atmospheric corrosion, especially when plated copper or bronze undercoats are used. This alloy has a low coefficient of friction. Deposits are solderable, hard (650—710 HV 5Q), act as etch resists, and find use in printed circuit boards, watch parts, and as a substitute for chromium in some applications. The rose-pink color of 65% tin is attractive. In marine exposure, tin—nickel is about equal to nickel—chromium deposits, but has been found to be superior in some industrial exposure sites. Chromium topcoats increase the protection further. Tin—nickel deposits are brittle and difficult to strip from steel. Temperature of deposits should be kept below 300°C. 

The film of magnesium hydroxide formed can give rise to passivity. This is attacked by anions such as chloride, sulfate and nitrate. The passive film formed gives reasonable protection from corrosion in rural, marine and industrial atmospheres, as evidenced by the corrosion rate data given in Table 4.69. It is obvious from the data that the corrosion performance of magnesium alloy lies between aluminum and carbon steel. 

Materials such as metals, alloys, steels and plastics form the theme of the fourth chapter. The behavior and use of cast irons, low alloy carbon steels and their application in atmospheric corrosion, fresh waters, seawater and soils are presented. This is followed by a discussion of stainless steels, martensitic steels and duplex steels and their behavior in various media. Aluminum and its alloys and their corrosion behavior in acids, fresh water, seawater, outdoor atmospheres and soils, copper and its alloys and their corrosion resistance in various media, nickel and its alloys and their corrosion behavior in various industrial environments, titanium and its alloys and their performance in various chemical environments, cobalt alloys and their applications, corrosion behavior of lead and its alloys, magnesium and its alloys together with their corrosion behavior, zinc and its alloys, along with their corrosion behavior, zirconium, its alloys and their corrosion behavior, tin and tin plate with their applications in atmospheric corrosion are discussed. The final part of the chapter concerns refractories and ceramics and polymeric materials and their application in various corrosive media. 

Atmospheric Atmospheric corrosion due to the combined effects of rain and the deposition of salt and other pollutants will affect most equipment. Corrosion occurs while the metal surface is wet, and is strongly influenced by the composition of deposits (such as sulfates from industrial atmospheres and chlorides from marine atmospheres). External corrosion of steel and stainless steel process equipment beneath thermal insulation and fireproofing is of particular concern. 

At exposure of steel in heavily polluted industrial atmosphere the corrosion rate on the upper side of steel panels exposed at 45° inclination was only 37 per cent of the total corrosion. In clean air, by contrast, the corrosion effect of rain was predominant and the upper sides of the test panels corroded faster than the undersides ( 6). The atmospheric corrosion of steel proceeds in local cells, where the sulphate nests acts as anodes. This may be the explanation why the washing effect of rain prevails in polluted atmospheres, as rain water may wash away sulphates from the nests. 

From the practical and economic point of view atmospheric corrosion is closely associated with centers of population. Three factors here coincide high pollution level, high density of population, which in turn means great use of materials. The rate of atmospheric corroion decreases sharply with increasing distance from the emission source. This may be illustrated by the corrosion of carbon steel as function of the distance from the stack of a polluting industry in Kvarntorp, see FIG.8 (26). 

Indoors NO in combination with SO has a synergistic corrosive effect on electrical contact matefials like gold plating and copper and may also be important for other materials like carbon steel at storage in industrial atmospheres. 

For zinc surfaces, the rate of accumulation of substances is expected to affect the rate of corrosion of the metal. For aluminum in most environments, the passive surface oxide will protect the metal from further significant corrosion. Any interactions that occur on the surface will be among the accumulated substances and perhaps with the outermost layer of metal oxide. Interestingly, a few materials perform better in industrial atmospheres than in clean environments. Suzuki, et aL (17), have discussed the fact that weathering steel utilizes atmospheric pollutants, particularly sulfur dioxide gas, to form protective layers. 

The shiny appearance, low-weight per volume, favorable mechanical properties such as material strength, ease of forming, and handling are some favorable features of aluminum and hence its use in the food industry. In addition, aluminum has a better corrosion resistance than carbon steel as it readily forms a protective film that prevents further atmospheric corrosion. Aluminum is also lighter than stainless steel and hence its use in beverage cans. 

Aluminium in contact with structural steel. Although steel corrodes faster than aluminium in environments such as seawater, marine and industrial atmospheres, water containing SO2, and soft water, aluminium is the material that primarily corrodes when these two materials are galvanically coupled in the mentioned environments. The reason for this apparent paradox is that the corrosion potential of... 

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