Pitting critical temperature tests

Table 4.14 Critical temperatures in some pitting and crevice corrosion tests...

ASTM Standard G 150, Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels. 

ASTM G 150 (Standard Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels) describes in detail how to perform such experiments. The solution prescribed is 1 M NaCl, the suggested applied potential is +700 mV(SCE) and the starting temperature is deemed to be 0°C, with the temperature being increased at l°C/min. The CPT is defined as the temperature at which the current exceeds 100 pA/cm and remains so for greater than 1 min. The test method clearly states that alternative potentials can be used if they are within the range for which the CPT is potential independent. 

Kane, R. D., Wilhelm, S. M., and McIntyre, D. R., Application of the Critical Pitting Temperature Test to the Evaluation of Duplex Stainless Steel, Corrosion Testing and Evaluation Silver Anniversary Volume. ASTM STP 1000, R. Baboian and S. W. Dean, Eds., ASTM International, West Conshohocken, PA, 1990, pp. 289-302. 

Determination of the critical pitting or critical crevice temperature is often used as an alternative to comparing the severity of attack on several samples [2f-23], Procedures for this testing are described in Methods C and D of ASTM G 48. Specimens are immersed for a given period (e.g., 72 h) at a temperature that is not expected to cause attack. Fresh specimens are then exposed at 5°C increments for the same duration until the temperature at which attack occurs (critical temperature) is determined. For many materials, the critical temperatures of replicate specimens fall within a 5°C variation. Retesting of the same specimen can affect the critical temperature [24]. Testing may be initiated just below the critical temperature, if it can be estimated. If not, this technique can require many specimens and a few weeks to find the critical temperature. 

Critical pitting temperature can be rapidly obtained using electrochemical equipment to maintain a preset potential, increase temperature and detect the onset of corrosion by monitoring corrosion current [24], In ASTM G 150, Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels, a pre-set potential provides consistent conditions and a potential-independent critical pitting temperature. Sensitive detection of attack using the current allows an accelerated evaluation. 

The materials Monit and Sea-Cure are characterised by good resistance to pitting, crevice and stress corrosion cracking in seawater. The critical pitting corrosion temperature in the FeCls test is 328 K (55 °C) and the critical crevice corrosion temperature is 318 K (45 °C). In Table 36, the pitting potentials of the two superferrites and the austenitic steels 1.4539 (SAE 904 L, XlNiCrMoCu25-20-5) and X3CrNiMol7-13-3 (SAE 316,1.4436) measured in 5% NaCl solution are presented. 

Critical pitting temperature test Potentiostatic 200 mV (°C) Potentiostatic 350 mV (°C) Critical crevice temperature test Potentiostatic 200 mV (°C) Potentiostatic 350 mV (°C)... 

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. 

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. 

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. 

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