Pitting temperature

The critical pitting temperature (CPT) is widely used as a measure of the resistance of stainless steel against pitting attack. Various methods for determination of the CPT are described here, special attention being given to the choice of test potential for the control of stainless steel quality. 

More often the passive layer is broken down locally and then the steel is said to be attacked by localized corrosion, the most important forms being pitting, crevice corrosion, and corrosion cracking. Most often the localized corrosion is caused by halogen ions such as chloride, bromide, and iodide. Pitting or pitting corrosion is seen as small pinholes on the surface of the steel. This section describes electrochemical instrumental methods to investigate and measure this form of corrosion attack. 

The mechanism of pitting is usually considered as involving two phases (1) the nucleation phase (initiation phase) when the passive layer 

It is not simple to give a measure of the resistance to pitting of a particular stainless steel, because it depaids on many factors associated with the actual environment. The three main factors are potential, temperature, and concentration of initiating ions. 

Experimental methods exist to determine a critical value for any one of these factors when the others are held constant. However, parameters such as pH, concentration of inhibiting ions (e.g., SO4 ), dissolved gas, test area, flow rate, and surface finish influence the resistance to pitting.  

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In the early 1970s, Brigham and Tozer were the first to make a systematic study, using temperature as the variable, of the connection between potential, temperature, and pitting corrosion (Fig. 12). They argued that in principle a critical pitting temperature should exist, but the data obtained showed a transition over a range of temperatures. The sharp transition was demonstrated experimentally by Quarfort in 1989. 

Figure 12. The limits of pitting as a function of potential and temperature according to the experimental results of Brigham and Tozer. CPT, critical pitting temperature.

Using this method, it takes many days to determine a single reliable CPT value. Salinas-Bravo and Newman published in 1994 what they called An alternative Method to Determine Critical Pitting Temperature of Stainless Steels in Ferric Chloride solution. ... 

Phase Inversion Temperature (PIT) Temperature at which the hydrophilic and oleophilic natures of a surfactant are in balance. As temperature is increased through the PIT, a surfactant will change from promoting one kind of emulsion, such as OAV, to another, such as WO. 

Moayed, M.H., Laycock, N.J. Newman, R.C., Dependence of the critical pitting temperature on surface roughness, corrosion science, 45, pp 1203-1216, 2003. 

Crevice corrosion is, of course, dependent on temperature. No crevice corrosion is found to occur below a certain temperature called the critical crevice corrosion temperature, which is similar to the critical pitting temperature mentioned earlier. The critical crevice corrosion temperature has been used as a measure to evaluate metallic materials for the susceptibility to crevice corrosion [63],... 

Effects of Temperature on Pitting the Critical Pitting Temperature... 

Fie. 7.21 Pitting temperature range of type 31 7L stainless steel exposed to chloride solutions of different oxidizing power for 24 and 66 h. Dashed lines are based on potentiodynamic data in Fig. 7.19. Redrawn from... 

Fig. 7.22 Correlation between the critical pitting temperature and critical pitting potential of 1 7 high-performance alloys. The alloys are ...

ASTM G150 is a new standard for the determination of critical pitting temperature (GPT) [4]. That standard describes a flushed port cell, in which the sample is pressed against an o-ring attached to a cell. 

G150-99, Standard test method for electrochemical critical pitting temperature of stainless steel. Annual Book of ASTM Standards, ASTM International, Philadelphia, Pa., 2000, p. 638, Vol. 3.02. 

V.M. Sahnas-Bravo, R.C. Newman, An alternative method to determine critical pitting temperature of stainless steek in ferric chloride solution, Corros. Sci. 36 (1994) 67—77. 

R. Qvarfort, Critical pitting temperature measurements of stainless steek with an improved electrochemical method, Corros. Sci. 29 (1989) 987—993. 

M.H. Moayed, R.C. Newman, Evolution of current transients and morphology of metastable and stable pitting on stainless steel near critical pitting temperature, Corros. Sci. 48 (2006) 1004—1018. 

The steel that is ranked highest in relation to pitting and creviee eorrosion in Table 10.7 is also represented in Figure 10.4 (20-25-4.5, i.e., 20Cr25Ni4.5Mo). The figure shows that the newer austenitic steels 20-18-6 and 24-22-7 as well as the ferritic-austenitic steel 25-7-4 are even better. The eolunms in the figure indieate critieal pitting temperature and critieal crevice corrosion temperature for rolled products as tested in a 6% FeCU solution. 

Figure 10.4 Critical pitting temperature (CPT) and critical crevice corrosion temperature (CCT) for various stainless steels in 6% FeCb solution [10.9]. The figures below the columns show the contents of Cr, Ni and Mo, respectively (compare Tables 10.5 and 10.6).

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