This paper describes the temperature dependence of the pitting corrosion resistance of a number of stainless steels, Fe-Cr-Ni and Fe-Cr-Ni-Mo materials in a series of high-temperature high-pressure chloride solutions, with or without addition of various (inhibitive) anions: SO4=, HCO3-, PO43-, OH-, etc.

Tests have mainly been carried out by means of potentiodynamic electrochemical methods in a recirculating autoclave system. Test temperatures varied between 50 and 300°C, with pressures up to 120 bar (12 MPa). After the tests, samples have been analyzed by means of various microscopic and surface-analytical methods (optical microscopy, SEM, EDX, AES-SAM, etc.).

The different results show some remarkable tendencies: In the lower temperature range, i.e. between 50 and ca. 150°C, the results of the electrochemical tests are fully in line with known literature data and practical experience, and show a sharp decrease of the pitting potentials (Ep) with increasing temperatures. However, for most test solutions this trend gradually diminishes as temperature rises above ca. 150 to 175°C, and no further decrease of pitting potentials is observed above 200°C, in none of the test solutions.

At these higher temperatures, Ep-values may even shift again to higher values and the typical pitting morphology may change into a more generalized type of corrosion. This means that pitting potentials (and also the corresponding repassivation potentials) show a minimum value at about 175-200°C. This is often accompanied by the formation of thick oxide layers and more shallow types of (localized) attack, including under-film of ‘filiform’-like types of corrosion, etcetera. These fairly remarkable results are in line with some older data for stainless steel Type 304 in high-temperature high-pressure aqueous environments.

A possible explanation might be related with transpassive dissolution phenomena of the Fe-Cr-Ni-(Mo) alloys and the effect on their protective surface films. Also the chloride reactions related to the pitting initiation process, as well as the ‘occluded cell’ pit growth mechanisms, might change at these higher temperatures. This presentation will discuss this in some more detail.

Subject
Pits, Materials, Corrosion resistant alloys, Stainless alloys, Nickel based alloys, Molybdenum alloys, Autoclaves, Chloride solutions, Corrosion attacks, Pitting, Stainless steel, Chlorides, Electrode potential
Keywords:
pitting, geothermal, high-temperature, stainless steel, nickel alloys
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2017
Association for Materials Protection and Performance (AMPP)
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