The comparison of the pitting resistance of alloys with high corrosion resistance can be challenging, as CPT-values (critical pitting temperatures) are often above the boiling point of the test solutions. For high-nickel alloys, the use of potentiostatic electrochemical methods at increasing temperatures can also be questionable, as this alloy type often shows high or continuously increasing current densities with increasing temperature, rather than the desired distinct current peak at pitting. The applied electrochemical potential must also be proven to be below the transpassive potentials of the tested alloys.
A new electrochemical method has been developed in which CPT-values could be determined of alloys UNS N08935, UNS N06625, UNS N06455 and UNS N06985. The electrochemical potential used is shown to be below transpassive potentials. The current density versus temperature curves were analyzed and showed low passive current densities up to temperatures where pitting started.
CPT-values of the tested alloys were around 90°C for UNS N08935, UNS N06625 and UNS N06455 and 55°C for UNS N06985.
Laboratory CPT-tests often have the benefit of being quick and easy to perform and can give important information of the pitting resistance of different alloys. Tests of pitting resistance are also frequently used for delivery testing, as a reduction in the CPT-value can identify defects in the materials. In this case a pass or fail-test at one temperature is typically done, instead of determining a CPT-value.
Additionally, CPT-values can be used to differentiate between the pitting resistance of different alloys. This can be valuable at materials selection and may assist in evaluating life-time cost related to properties. The actual conditions at the place of intended use must be well known, as for instance the pH-value or contaminants may influence the corrosion resistance.
Some problems may be encountered when attempting to determine CPT-values for certain alloys or using specific methods. It may, for instance, be difficult to find CPT-values for alloys with very high pitting resistance, as pitting may initiate only above the boiling points of test solutions. ASTM(1) G48 was developed using a maximum temperature of 85°C.1 Changed test conditions at increased temperatures have been reported, especially of the pH-value, the potential and formation of deposits on sample surfaces.2,3,4 Another standardized method, ASTM(1) G150, uses an applied electrochemical potential of 700 mVSCE (SCE: saturated calomel electrode) and a critical current density for CPT of 0.1 mA/cm2.5 It has the advantage of being quick, as CPT is determined without iterative procedure and each test typically has a duration of a few hours. On the other hand, the test was developed for stainless steels and its use for nickel alloys is questionable, primarily due to their often lower transpassive potentials and potentially higher passive current densities. The 1M NaCl-solution used in ASTM(1) G150 is also too weak for highly pitting resistant alloys to obtain pitting.