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- Biodegradation (1)
- Corrosion (1)
- Critical pitting temperature (1)
- Electrochemical noise (1)
- Electrochemistry (1)
- Magnesium (1)
- Magnesium alloys (1)
- Ni Cr Mo alloys (1)
- Plasma-chemical oxidation (1)
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The determination of critical pitting temperatures (CPT) in various test solutions like ferric chloride solution according to ASTM G48 or 'Green Death' solution is a common test method for the comparative assessment of the pitting corrosion resistance of highly alloyed steels and Ni–Cr–Mo alloys. In addition to the well-known disadvantages of standard methods, like long test times, subjective examination, and large scatter, for the highest alloyed Ni–Cr–Mo alloys no stable pitting corrosion can initiate even at the highest test temperatures. This paper describes the limitations of standard test methods and shows how these problems can be solved by an alternative test solution and an adjusted test method. By capturing and examining the current noise under potentiostatic conditions during continuous heating in a 4.5 M calcium chloride solution the transition from metastable to stable pitting corrosion as a criterion for CPT can be detected in a reproducible way.
The degradation behaviour of an Mg-1Ca alloy is investigated in vitro to figure out the possibilities of influencing the degradation behaviour of such an alloy by applying heat treatment as well as the use of a coating system based on plasma-chemical oxidation. It is shown that an optimised solution annealing (T4-heat treatment) can reduce the degradation rate while an additional ageing between 240 and 300 °C (T6-heat treatment) increases it. A coating generated by plasma-chemical oxidation reduces the degradation rate in the immersion test. Its effect is depending on the former heat treatment of the Mg-1Ca alloy as well as on the parameter during plasma-chemical oxidation.