Localized corrosion test techniques

CCS to dominate and the localized corrosion site to repassivate. In studying the localized corrosion test techniques, these three aspects of localized corrosion, the CCS, the separation of anode and cathode, and the stability of these processes, should be kept in mind. 

Even with effective corrosion test programs, many unexpected failures occur in chemical processes. There are several reasons. The corrosion behavior of a material is a combination of the property of the metal (composition and condition) and of the environment. Corrosion can take many forms, from general to localized. An acceptable rate of general corrosion may not translate into successful performance if there is also localized corrosion, such as pitting or stress corrosion cracking. There is no universal corrosion test for all situations, and sometimes a combination of corrosion test techniques may be required to raise one s level of confidence. 

As with all elec trochemical studies, the environment must be electrically conduc tive. The corrosion rate is direc tly dependent on the Tafel slope. The Tafel slope varies quite widely with the particular corroding system and generally with the metal under test. As with the Tafel extrapolation technique, the Tafel slope generally used is an assumed, more or less average value. Again, as with the Tafel technique, the method is not sensitive to local corrosion. 

The objectives of this chapter are to provide a basic explanation of the chemical and physical processes involved in localized corrosion and to explain the test techniques that are commonly used to determine the resistance of alloys to localized corrosion. 

Two other aspects of electrochemical phenomenology associated with localized corrosion should be appreciated before we discuss individual test techniques common observations during potentiostatic testing and common observations during open circuit testing. Careful interpretation of these tests can provide useful information on the processes that control localized corrosion. 

Galvanostatic methods for localized corrosion. At constant chosen currents, the evolution of potential as a function of time is recorded until the rate of change in potential with time approaches zero. This technique is under development for aluminum alloys in ASTM Gl4 as a test method for application to aluminum alloys. (Scully)14... 

HS Isaacs, B Vyas. Scanning reference electrode technique in Localized Corrosion. In F Mansfeld, U Bertocci, eds., Electrochemical Corrosion Testing. ASTM STP 727 3, 1981. 

Extensive testing on stainless steel mockups, fabricated using production techniques, has been conducted to determine the effect of various welding procedures on the susceptibility of unstabilized 300 series stainless steels to sensitization-induced intergranular corrosion. Only those procedures and/or practices demonstrated not to produce a sensitized structure are used in the fabrication of RCPB components. The ASTM standard A 708 (Strauss Test) is the criterion used to determine susceptibility to intergranular corrosion. This test has shown excellent correlation with a form of localized corrosion peculiar to sensitized stainless steels. As such, ASTM A 708 is utilized as a go/no-go standard for acceptability. 

Other types of specimen monitoring that can be performed by this technique are (1) visual observation of the development of localized corrosion phenomena with time, (2) monitoring of environmental crack initiation, and (3) crack growth rate measurements. In these cases, it is important to be able to provide for incoming light and for reflection of light from the specimen in order to visually observe the test specimen. Therefore, such measurements often require somewhat larger transparent ports and are usually limited to lower pressure systems. 

Although ASTM tests use temperature to accelerate attack during exposures, temperature can also be used in combination with electrochemical techniques. One example of this is the determination of critical pitting temperatures (CPT) [76,37,32] for alloy development or selection, or both. An anodic potential is applied to an electrode at low temperature (room or below) in the solution of interest and the temperature slowly increases in order to determine the temperature at which initiation of localized corrosion occurs, as signified by an increase in the current above some criterion. Such a method allows a quantitative ranking of materials in terms of resistance to pitting. 

The goals of this chapter were to give a brief overview of the exposure tests used to determine localized corrosion susceptibility, and to introduce a variety of electrochemical techniques that can provide important information concerning localized corrosion susceptibility. Although none of the... 

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