Average corrosion penetration rate

After icorr is evaluated by any one of the foregoing methods, use of one of the Faraday-law expressions (Table 6.2 and Chapter 4) leads to either the average corrosion intensity (Cl) or average corrosion penetration rate (CPR). If the corrosion process is uniform, these average values relate directly to the uniform surface dissolution rate. If, on the other hand, the corrosion process is localized, the actual corrosion intensity and corrosion penetration rate at local areas can be orders of magnitude greater than the average values. 

It should be made clear that all the rates of rusting in the atmosphere just quoted, relate to average general penetration and take no account of pitting. Serious pitting of steel exposed to atmospheric corrosion is uncomm.on on simple test plates, but it may be necessary to allow for this in some practical cases, where local attack may be occasioned by faulty design and other factors. 

Corrosion of External Equipment, The results of corrosion tests on 1020 steel, in approximately 1 x 10 normal hydrogen peroxide (representative of the circulating solution) show an average penetration rate of 0.016 per month or about 0.19" per year. The heat exchanger tubes which probably would be used are only 0.083" in wall thickness. Obviously this ordinary steel cannot be used without corrosion protection of some sort. 

Maximum Penetration in mils (Imil = 0.001 in. = 0.025 mm) for Total Exposure Period Average corrosion rates in g d (gmd)... 

So far the discussion has been limited to a consideration of homogeneous metals in a uniform environment where corrosion will occur at the same rate over the whole surface. This situation seldom constitutes a major corrosion hazard it is often possible to plan remedial work when armed with a knowledge of the uniform averaged corrosion rate expressed as a weight loss (e.g. mg dm day mdd) or a penetration rate, e.g. mm year r mpy. 

The averaged penetration rate Axjt may be calculated from a knowledge of the density of the metal p, again with the assumption of uniform corrosion ... 

For instance, the value of 4orr= 10 cm corresponds to an average penetration rate of 0.1 mm year. The value 10 pA cm" is an approximate criterion for determination of whether or not an aluminum alloy is corrosion-resistant (Masuko, 1978). 

Measurement of the average corrosion rate, per square centimeter of the sample, yields only part of the pertinent information, often a small part Consider a piece of metal corroding in a given environment at a rate of 0.1 mmy . This is a rather low rate, which may not worry the designer too much, if corrosion were uniform. Adding 1 mm to the wall thickness of a pipe, for example, would provide an additional service life of 10 years. On the other hand, if corrosion occurs on 1% of the surface, the same average corrosion rate would correspond to a penetration of 10 mm y" and one could not very well increase the wall thickness to 100 mm to provide a service life of 10 years. 

This test, which is frequently referred to as the Huey test, was first described and used by W. R. Huey in 1930, and since that time it has had wide application, particularly in the USA. The test consists of exposing the specimens (20-30 cmin fresh boiling 65% HNO3 (constant boiling mixture) for five successive periods of 48 h each under a reflux condenser. The specimens are cleaned and weighed after each period, and the corrosion rate (as a rate of penetration) is calculated for each period of test and for the average over the five periods corrosion rates are expressed as mm/y. The reason for the... 

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