Corrosion environmental factors

Unprotected steel corrodes at a rate which is generally assumed to be 0.1 to 0.2mm per annum. Factors that influence the actual rate of corrosion include the maintenance program applied by the owner - particularly preservation of protective coatings, efficiency of cathodic protection systems in ballast tanks, corrosive properties of the cargo carried and environmental factors such as temperature and humidity. Under extreme conditions it has been known for the annual rate of corrosion on unprotected steel exposed on both surfaces to approach 1mm. 

The environment plays several roles in corrosion. It acts to complete the electrical circuit, ie, suppHes the ionic conduction path provide reactants for the cathodic process remove soluble reaction products from the metal surface and/or destabili2e or break down protective reaction products such as oxide films that are formed on the metal. Some important environmental factors include the oxygen concentration the pH of the electrolyte the temperature and the concentration of anions. 

In general, as the corrosiveness of the environment increases, the rate of crack growth also increases. Environmental factors constituting corrosiveness vary with the metal under consideration. 

Erosion-corrosion is a fairly complex failure mode influenced by both environmental factors and metal characteristics. Perhaps the most important environmental factor is velocity. A threshold velocity is often observed below which metal loss is negligible and above which metal loss increases as velocity increases. The threshold velocity varies with metal and environment combinations and other factors. 

The percentage of failures that occur solely from material defects is quite low—less than 1% of all failures. What is more common is that defects may act in conjunction with specific environmental factors to produce failures, such as cracking or localized corrosion. 

General description. In incomplete fusion, complete melting and fusion between the base metal and the weld metal or between individual weld beads does not occur (Fig. 15.8). Incomplete fusion that produces crevices or notches at surfaces can combine with environmental factors to induce corrosion fatigue (Chap. 10), stress-corrosion cracking (Chap. 9), or crevice corrosion (Chap. 2). See Fig. 15.9. 

Evaluate potential for external corrosion from environmental factors such as chloride bearing insulation, chemical spills, sea mist, road salt, etc. 

Soil has been defined in many ways, often depending upon the particular interests of the person proposing the definition. In discussion of the soil as an environmental factor in corrosion, no strict definitions or limitations will be applied rather, the complex interaction of all earthen materials will come within the scope of the discussion. It is obvious only a general approach to the topic can be given, and no attempt will be made to give full and detailed information on any single facet of the topic. 

Soil is distinguished by the complex nature of its composition and of its interaction with other environmental factors. No two soils are exactly alike, and extremes of structure, composition and corrosive activity are found in different soils. Climatic factors of rainfall, temperature, air movement and sunlight can cause marked alterations in soil properties which relate directly to the rates at which corrosion will take place on metals buried in these soils. 

As with other factors, no direct statements can be made relating the reaction of a soil to its corrosive properties. Extremely acid soils (pH 4 0 and lower) can cause rapid corrosion of bare metals of most types. This degree of acidity is not common, being limited to certain-bog soils and soils made acid by large accumulations of acidic plant materials such as needles in a coniferous forest. Most soils range from pH5 0 to pH8 0, and corrosion rates are apt to depend on many other environmental factors rather than soil reaction per se. The 45-year study of underground corrosion conducted by the United States Bureau of Standards included study of the effect of soils of varying pH on different metals, and extensive data were reported. 

Chlorides are often found as the salt aerosols of the atmosphere, and consequently may strongly influence the corrosion performance of structures and plant, particularly in marine or coastal situations. This influence on corrosivity reduces proportionately with distance from the seawater surface, though local environmental factors such as prevailing wind direction, level... 

The results obtained with this equipment show that the corrosion rate in the rig is about four times that encountered in an industrial UK atmosphere. This acceleration, however, is not achieved by accentuating any of the environmental factors, but rather by holding them near to the worst natural conditions for as long as possible. The procedure used ensures that the rust film is completely dried for short periods, thus simulating the conditions that bring out the beneficial effects of protective rust films on the steels under study. 

When sensitive metals are exposed to certain environmental factors or tensile stress, corrosion can occur. Exposure to only a few parts per million of a corrosive agent can initiate stress corrosion in some metals. High concentrations are often not necessary. Also, temperature and pH can be influencing factors. 

The four important areas of application of carbon steels are (i) atmospheric corrosion (ii) corrosion in fresh water (iii) corrosion in seawater and (iv) corrosion in soils. The atmospheric corrosion of steel is caused by major environmental factors such as (i) time of wetness as defined by ISO 9223-1992 (ii) sulfur dioxide in the atmosphere due to the combustion of fossil fuels and (iii) chloride carried by the wind from sea. The equations for corrosion rates of carbon steel by multiple regression analysis have been obtained.1... 

The environmental factors that influence the corrosion rate vary with the depth in seawater as detailed8 in Table 4.3. The variation of the factors in seawater at different global locations has been documented in the literature.9... 

Corrosive to eyes, skin, mucous membranes, and teeth. Causes upper respiratory irritadon that may seem to clear up, only to return in a few hours and more severely. Depending on environmental factors the vapor will consist of a mixture of the various oxides of nitrogen and nitric acid. Flammable by chemical reacdon with reducing agents. It is a powerful oxidizing agent. 

The influence of mainly SO on the corrosion rate of several materials has been shown in numerous national exposure programs. During the last decades a number of empirical relations have been derived from measurements of atmospheric corrosion rates of the most important structural metals and from measurements of environmental factors. The results are usually presented in form of equations including pollution and meteorological parameters (5.). 

The second step is the interaction of this corrosion product with environmental factors such as the weather, gaseous pollutants, and wet and dry deposition to produce, while conserving metal in the film, a modified or weathered corrosion product AB ... 

Environmental Factors Affecting Corrosion of Weathering Steel... 

Haynie, F.H. Environmental Factors Affecting the Corrosion of Weathering Steel. To be published in Proceedings of ACS Meeting, Degradation of Materials Due to Acid Rain, Arlington, Va., June 1985. 

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