Abstract
The corrosion resistance of a steel of type 18Cr2MoTi has been determined relative to that of well known, standardised stainless grades.
The resistance to general corrosion was evaluated in sulphuric, phosphoric, formic, acetic, lactic and oxalic acids. It has been established that the corrosion resistance of 18Cr2MoTi as of other ferritic steels is strongly dependent on the surface preparation of the specimen. The passivity is improved in the direction activated → ground → pickled. Accordingly, the acid resistance is better for 18Cr2MoTi than for Type 316 in the ground and pickled state, whereas in the activated state the resistance is inferior to that of the reference material when subjected to the strongest acids such as sulphuric, phosphoric and oxalic acid.
Intergranular corrosion tests of welds were performed in different media. The lowest stabilisation content of titanium to ensure freedom from intergranular corrosion is dependent upon the sensitisation circumstances and the test solution. If perfect TIG welding and the Strauss test are selected, a critical degree of stabilisation seems to be Ti = 7(C + N), whereas a higher value is necessary if the gas shield at welding is imperfect, or if a more severe corrosion medium is applied. If good fabrication practice is used a stabilisation degree of Ti = 10(C + N) should guarantee resistance to intergranular corrosion under all circumstances.Anodic polarisation measurements in a NaCl solution at 25–907°c have shown that 18Cr2MoTi has a better resistance to the initiation of pitting corrosion than has Type 316. In this case, also, the best result is obtained for 18Cr2MoTi with a pickled surface. Measurements of welds and heat-affected zones have shown that pickling after welding has considerable influence on the pitting corrosion resistance. This applies to both 18Cr2MoTi and Type 316.Tests of the resistance to crevice corrosion in water with different chloride contents revealed that, for a pickled surface, 18Cr2MoTi is better than Type 316 but it seems that the reverse is true for ground surfaces. After exposure to water containing 100 ppm Cl− and 9ppm O2 at 200°c, only crevice corrosion was noted for Type 430 and 18Cr2MoTi, whereas stress corrosion cracking took place on Type 304 and Type 316. At 300°e, Type 430 and 18Cr2MoTi were heavily attacked by pitting corrosion and intergranular stress corrosion cracking, respectively. An explanation for the last mentioned observation has not been found.Atmospheric exposure is being carried out in Stockholm and on Bohus–Malmön on the west coast of Sweden. At the latter test station a clear distinction in favour of 18Cr2MoTi compared with Type 304 has been noted for pickled and ground surface preparation, after 3 years exposure. Gravimetric oxidation tests in air have revealed a scaling temperature of100°c for 18Cr2MoTi, which is a higher value than for Type 304 and Type 316.
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