Cracking atmospheric corrosion

Excellent resistance to saltwater corrosion and biofouling are notable attributes of copper and its dilute alloys. High resistance to atmospheric corrosion and stress corrosion cracking, combined with high conductivity, favor use in electrical/electronic appHcations. 

Rolled or pickled steels corrode faster than machined or polished steels. The mill scale on the sample must be removed before exposure to avoid the possibility of mill scale acting as cathode. The mill scale may initially give protection against atmospheric corrosion, but scale tends to crack and spall under shock conditions, resulting in bare spots on the sample exposed to the atmosphere. The vertical plates are attacked more slowly than plates at 45°. 

The accumulation of salts within the concrete pore structure can also lead to the corrosion of reinforcing steel, the fourth form of deterioration identified above. This corrosion is accompanied by an increase in the volume of the steel, which eventually causes the concrete to crack and spall. In discussing the atmospheric corrosion of concrete reinforcements, Skoulikidis (21) notes "The increase of atmospheric pollution Intensifies the corrosion tendency of the reinforcements in the atmosphere. The cracking of the concrete was observed more frequently with an increase of the atmospheric pollution (SO2, CO2, NH3, NOx> etc.) and the acceleration of the corrosion by the formation of a more conductive environment, that also chemically attacks the metals."... 

Corrosion under paint films and atmospheric corrosion. The loss of metal due to corrosion can be observed with absorption contrast, and any changes in the paint film (e.g., cracks, swelling with water) could be observed with phase contrast imaging. If suitable ions are introduced, their distribution could be tracked with energy difference imaging, described above. For atmospheric corrosion, salt droplets and crystallization of salts and corrosion products could be observed and correlated with pit development. 

Of interest in this connection is the fact that atmospheric corrosion mainly occurs in the form of a more or less uniformly distributed scar-shaped erosion. However, other forms of corrosions are also encountered. Thus, for instance, the impact of a humid atmosphere on zinc alloys may produce intercrystalline corrosion, and the impact of a humid atmosphere containing NH3 may lead to stress cracks in brass components. 

Specific tests frequently used are (a) neutral 5 % Sodium Chloride salt spray (ASTM B 117, Test Method of Salt Spray (Fog) Testing), (b) 3.5 % Sodium Chloride by alternate immersion (ASTM G 44, Practice for Evaluating Stress Corrosion Cracking Resistance of Metals in 3.5 % Sodium Chloride Solution), and (c) exposure to various outdoor atmospheres. Guidelines for outdoor exposure are contained in ASTM G 50, Practice for Conducting Atmospheric Corrosion Tests on Metals. Generic types of atmospheres used are seacoast, industrial, urban, and rural. Sometimes specific geographical locations or local chemical conditions are important because they can produce unique results [2i],... 

Atmospheric corrosion is considered that which occurs from exposure of metals to atmospheric conditions, including precipitation and dew. Atmospheric corrosion damage includes general attack, cracking, and localized corrosion. 

The copper-nickel alloys are referred to as cupronickels and have nickel as the principal alloying ingredient. Of all the copper alloys, the cupronickels are the most resistant to stress corrosion cracking in ammonia and ammon-ical environments. These alloys do not find many applications to resist atmospheric corrosion because of their cost. They would be most suitable for marine atmospheres. 

In the natural atmosphere noble metal coatings, such as chromium, nickel, and their alloys, form passive films on carbon steel surfaces. A defect-free continuous layer of chromium provides excellent protection for a carbon steel substrate. However, deposited layers of chromium and nickel commonly have some defects, such as cracks. The corrosion rate of the carbon... 

Fig. 1Ql20 a simplified case of atmospheric corrosion due to a break (e.g. a icracch or crack) in a paint film. 

Zinc (1-8%) is the major alloying element. These are high strength heat treatable alloys mainly used in airframe structures. Among the Al-Zn-Mg-Cu added alloys, they provide the highest strength. These alloys should be coated before exposure to atmosphere because of their low resistance to atmospheric corrosion. They also exhibit reduced resistance to stress corrosion cracking. 

The formation of corrosion products, the solubility of corrosion products in the surface electrolyte, and the formation of passive films affect the overall rate of the anodic metal dissolution process and cause deviations from simple rate equations. Passive films distinguish themselves from corrosion products, in the sense that these films tend to be more tightly adherent, are of lower thickness, and provide a higher degree of protection from corrosive attack. Atmospheric corrosive attack on a surface protected by a passive film tends to be of a localized nature. Surface pitting and stress corrosion cracking in aluminum and stainless alloys are examples of such attack. 

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