Corrosion testing objectives

Corrosion Testing Objectives of SCC testing are (i) determination of the risk of SCC for a given application and comparison of alloys (ii) examination of the effects of chemical composition, metallurgical processing, fabrication practices for structural components (iii) evaluation of protective systems and prediction of service life (iv) development of new alloys, which are less expensive, and offer a longer, safer, and efficient performance for chosen environments (v) evaluation of claims for SCC performance of improved mill products. 

The CMT method was primarily developed to replace traditional corrosion tests with a faster, quantitative test. Since the course of corrosion with time is continuously shown on the screen, it is easy to see if and when a stable situation has been established, resulting in a constant rate of corrosion. The rate of corrosion at that stage can often serve as a safe measure of the lifetime of the object. An assessment of quality on the basis of such a measurement can be made in a much shorter time than by a chamber test. In the latter, a good quality product should typically have a time to failure of more than 100 h, whereas a CMT test of a similar product mostly shows a stable, constant rate of corrosion (on the order of 5-10 fiAJcrn ) after less than 2 h. 

Corrosion Testing. Typical objectives of SCC testing programs include ... 

Predictions of the corrosion performance should be obtained from published data and through testing.The essential requirements of accelerated testing are that the acceleration should produce the same mode of failure and reflect at least a known order of resistance of some alloys in service media.168 The most common approaches employed to achieve testing objectives in SCC are the use of high stresses, slow continuous straining, precracked specimens, higher concentration of species in the test environment than in the service environment, increased temperature, and electrochemical stimulation.169 For electrochemical corrosion, the properties of the medium at the interface should be considered in accelerated tests. 

In this chapter, first a brief introduction regarding general objectives, methods and procedures of corrosion testing is given. This is followed by a presentation of the most important principles of eleetroehemieal testing, and finally some aetual methods of monitoring and inspeetion are reviewed. For further study, referenee is made to References [9.1-9.4] and other sourees indicated at the respeetive places in the text. 

Clearly defined goals and objectives are needed if corrosion tests are to produce results that affect materials selection, and methods of corrosion protection and corrosion control. Goals and objectives define test purpose and what is to be achieved. Economic considerations are always a factor in defining goals and objectives, along with other considerations (workforce, public safety, and environment). Figure 2 shows some of the quality criteria that affect plant economics in the CPI. The criteria have been grouped... 

Standard corrosion tests such as those developed by ASTM, NACE, and ISO often will address the technical requirements established in the goals and objectives though, in some cases, they may need to be modified. The quality of data produced by standard tests has been well established by critical evaluation and review, and results can readily be compared to others obtained using the same test. Even the highly specialized techniques used in fundamental corrosion research, while not especially amendable to standardization, eventually achieve codification through continued publication and use [9,17-20. ... 

Many of the measurements involved in corrosion tests are defined in standards to establish precision, accuracy, representativeness, and comparability [10-14], Analytical techniques and measurements used to characterize the environment and corrosion product chemistry, and to evaluate coatings properties and inhibitor performance, are largely covered by good laboratory practices" standards [45]. In addition, the requirements for certain analytical and environmental measurements used in regulatory efforts have been established in the Code of Federal Regulations (CFR) [47]. The corrosionist should become familiar with and use these sources of information as they apply to the goals and objectives of the study. Table 5 lists some of the ASTM standards that are more widely used by corrosionists. 

One of the most important parts of corrosion testing is the preparation of the specimen or part to be tested. Although it is desirable to use the same surface conditions for the tests as those found in service, this is not always possible because of the variability of the material s surface in service, but it should always be a desirable objective. 

This practice is intended primarily where the test objective is to determine the relative stress corrosion cracking susceptibility of different brasses under the same or different stress conditions or to determine the absolute degree of stress corrosion susceptibility, if any, of a particular brass or brass component under one or more specific stress conditions. The tensile stresses may be known or unknown, applied or residual. 

The objectives of corrosion testing and associated monitoring of chemical, stress, and thermodynamic conditions in steam generating cycles are ... 

Basic objectives of corrosion testing in cable plants 19... 

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