Atmospheric corrosion, metals

Because this is an electrochemical process, an electrolyte must be present on the surface of the metal for corrosion to occur. In the absence of moisture, which is the common electrolyte associated with atmospheric corrosion, metals corrode at a negligible rate. For example, carbon steel parts left in the desert remain bright and tamish-free over long periods. Also, in climates where the air temperature is below the freezing point of water or of aqueous condensation on the metal surface, rusting is negligible because ice is a poor conductor and does not function effectively as an electrolyte. 

Atmospheric corrosion results from a metal s ambient-temperature reaction, with the earth s atmosphere as the corrosive environment. Atmospheric corrosion is electrochemical in nature, but differs from corrosion in aqueous solutions in that the electrochemical reactions occur under very thin layers of electrolyte on the metal surface. This influences the amount of oxygen present on the metal surface, since diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Atmospheric corrosion rates of metals are strongly influenced by moisture, temperature and presence of contaminants (e.g., NaCl, SO2,. ..). Hence, significantly different resistances to atmospheric corrosion are observed depending on the geographical location, whether mral, urban or marine. 

The consumption of oxygen due to atmospheric corrosion of sealed metal tanks may cause a hazard, due to oxygen-deficiency affecting persons on entry. 

Critical Humidity—the relative humidity (RH) at and above which the atmospheric corrosion rate of a metal increases significantly. 

BS2569 Sprayed Metal Coatings. Part 1 Protection of Iron and Steel by Aluminum and Zinc Against Atmospheric Corrosion. ... 

The main factor in causing filiform corrosion is the relative humidity of the atmosphere, and if this is below 65% (the critical relative humidity for the atmospheric corrosion of most metals, see Section 2.2) it will not occur. As the relative humidity increases the thickness of the filaments increases at 65-80% relative humidity they are very thin, at 80-95% relative humidity they are much wider and at approximately 95% relative humidity they broaden sufficiehtly to form blisters. 

Metals are more frequently exposed to the atmosphere than to any other corrosive environment. Atmospheric corrosion is also the oldest corrosion problem known to mankind, yet even today it is not fully understood. The principal reason for this paradox lies in the complexity of the variables which determine the kinetics of the corrosion reactions. Thus, corrosion rates vary from place to place, from hour to hour and from season to season. Equally important, this complexity makes meaningful results from laboratory experiments very difficult to obtain. 

However, the object of this section is to outline the principles which govern atmospheric corrosion, and the emphasis is placed on metals whose atmospheric corrosion is of economic importance. These include iron and steel, zinc, copper, lead, aluminium and chromium. 

However, in this section emphasis is placed upon damp and wet atmospheric corrosion which are characterised by the presence of a thin, invisible film of electrolyte solution on the metal surface (damp type) or by visible deposits of dew, rain, sea-spray, etc. (wet type). In these categories may be placed the rusting of iron and steel (both types involved), white rusting of zinc (wet type) and the formation of patinae on copper and its alloys (both types). 

Symposium on Atmospheric Corrosion of Non-Ferrous Metals, Amer. Soc. Test. Mat., 58th Annual Meeting, June 29 (1955). Spec. Tech. Publn. No. 175, 141-158... 

Because cast iron components are normally very heavy in section, the relatively low rates of attack associated with atmospheric corrosion do not constitute a problem and little work has been carried out on the phenomenon. A summary of some of the data available is given in Table 3.42. The most extensive work in this field was initiated by the A.S.T.M. in 1958 and some of the results produced by these studies are quoted in Table 3.43. It will be noted that there is a marked fall in corrosion rate with time for all the metals tested. 

The purity of the zinc is unimportant, within wide limits, in determining its life, which is roughly proportional to thickness under any given set of exposure conditions. In the more heavily polluted industrial areas the best results are obtained if zinc is protected by painting, and nowadays there are many suitable primers and painting schemes which can be used to give an extremely useful and long service life under atmospheric corrosion conditions. Primers in common use are calcium plumbate, metallic lead, zinc phosphate and etch primers based on polyvinyl butyral. The latter have proved particularly useful in marine environments, especially under zinc chromate primers . 

In principle, cathodic protection can be used for a variety of applications where a metal is immersed in an aqueous solution of an electrolyte, which can range from relatively pure water to soils and to dilute solutions of acids. Whether the method is applicable will depend on many factors and, in particular, economics — protection of steel immersed in a highly acid solution is theoretically feasible but too costly to be practicable. It should be emphasised that as the method is electrochemical both the structure to be protected and the anode used for protection must be in both metallic and electrolytic contact. Cathodic protection cannot therefore be applied for controlling atmospheric corrosion, since it is not feasible to immerse an anode in a thin condensed film of moisture or in droplets of rain water. 

Zinc diffusion sherardisingY " is mainly used for protection of ferrous metals against atmospheric corrosion. It has, in some respects, properties related to other types of zinc coating such as galvanising, but owing to the small dimensional change involved, it is of particular value for the treatment of machined parts, bolts, nuts, etc. 

Atmospheric corrosion can be prevented by using volatile inhibitors which need not be applied directly to the surfaces to be protected. Most such inhibitors are amine nitrites, benzoates, chromates, etc. They are mainly used with ferrous metals. There is still some disagreement as to the mechanism of action. Clearly, any moisture that condenses must be converted to an inhibitive solution. There is no doubt that the widely used volatile inhibitors are effective in aqueous solutions containing moderate... 

The relative susceptibility of metals to atmospheric corrosion varies widely with the type of contaminant, e.g. zinc and cadmium, two metals that are used for the protection of steel in exposed environments, are both rapidly attacked by organic acidson the other hand, aluminium alloys resist attack by organic acids but may be rapidly corroded by chlorides, especially at crevices or areas of contact. 

Practice for making and using U-bend stress corrosion test specimens Recommended practice for laboratory immersion corrosion testing of metals Method for vibratory cavitation erosion test Practice for recording data from atmospheric corrosion tests of metallic-coated steel specimens... 

Metals and alloys removal of corrosion products from corrosion test specimens Metals and alloys atmospheric corrosion testing general requirements for held tests Corrosion of metals and alloys. Classification of corrosivity of atmospheres Corrosion of metals and alloys. Guiding values for the corrosivity categories of atmospheres... 

Interest in this metal comes from its remarkable inertness to atmospheric corrosion. Also, it is very hard and thus it forms an ideal protective coating. On the basis of its E° (1.18 volts higher than hydrogen) we expect chromium to be quite reactive in fact, it is vigorously reactive with... 

Molybdenum oxidizes at high temperatures but not at room temperatures. It is insoluble in acids and hydroxides at room temperatures. At room temperatures, all three metals (chromium, molybdenum, and tungsten) resist atmospheric corrosion, which is one reason chromium is used to plate other metals. They also resist attacks from acids and strong alkalis, with the exception of chromium, which, unless in very pure form, will dissolve in hydrochloric acid (HCl). 

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