1174 Service and Field Practice

Like the above laboratory-based tests, in-service and field tests are designed to evaluate a range of material degradation processes including erosion-corrosion, stress corrosion, and conventional corrosion processes. In addition information is required concerning material performance within a wide range of operating environments, including atmospheric, sea and fresh waters, high-temperature, in vivo, soils and concrete. In this respect only a brief survey of the relevant test methods can be made here.

By far the most extensive type of test, arising from its simplicity is that of outdoor atmospheric coupon testing. These tests consist of exposing appropriate sized coupons (bare metal or coated) within the desired atmospheric conditions. Normally these are graded as urban, rural, and industrial; other classifications of

FIGURE 11.20 SRET maps illustrating shift in localized corrosion activity on a kelocouple joint due to changes in solution concentration: (a) = 4% solution and (b) = 0.4% solution. Note L = lacquer, S = steel, PA = pure aluminum, and A1A = aluminum alloy.

FIGURE 11.20 SRET maps illustrating shift in localized corrosion activity on a kelocouple joint due to changes in solution concentration: (a) = 4% solution and (b) = 0.4% solution. Note L = lacquer, S = steel, PA = pure aluminum, and A1A = aluminum alloy.

atmospheric conditions have also been derived [47]. Coupons are then examined periodically during which visual inspection is carried out and weight changes are monitored. This latter measurement provides an average corrosion rate for the selected metal/alloy in the given environment. Depending upon alloy type and the nature of the environment, corrosion rates can range from 0.5 ^m/year to 300 + ^m/year for highly concentrated chloride environments. Corrosion weight loss per unit area is based upon Faraday's law:

iMTS

where m is metal loss (g), i is corrosion current (A), M is molar mass of the metal (g/mol), T is time (s), S is area of metal involved (cm2), n is number of electrons released in the dissolution reaction, and F is Faraday's constant, 96487 C (As). It should be noted that this equation only applies to uniformly corroding surfaces. Corrosion coupons can also be used for assessing corrosion resistance in fresh and seawater environments.

Online monitoring techniques are becoming increasingly popular, particularly where chemical treatments such as inhibitor and biocide dosing is used to minimize corrosion activity. Potential monitoring is a common online technique whereby measurements of free corrosion potential, as a function of time, are made. Typical examples include measurements of potential on cathodically protected structures such as offshore oil platforms and reinforced steel within concrete structures.

Electrical resistance probes are used in a similar manner to coupons but without the need to remove the sample from the environment. This has two advantages: (i) all corrosion processes remain continuous and (ii) the method can be automated to provide information on "instantaneous" corrosion rate. However, information relating to whether corrosion is uniform or localized is not provided using this method. Similarly electrochemical probes are used to provide online information of corrosion rate, normally via measurements of polarization resistance. The use of probes, like that of laboratory tests, relies on the aqueous media being relatively conductive, otherwise errors are incurred due to an internal resistance (IR) drop across the electrodes. Electrochemical noise (EN) in the form of potential and current measurements is finding increasing application, although its extensive use is restricted due to the complex statistical analysis required to identify the nature of the corrosion. EN is used primarily for evaluating localized corrosion (pitting) and SCC [48, 49].

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