Corrosion inhibitive effect of potassium chromate and emulsion oils for diesel cooling systems were evaluated by recirculating coolant test, cavitation test and corrosion fatigue test.
The test results for the specimens made of Ni-Cr cast iron, Cu-alloy and Zn are summarized as follows;
1) Minimum level of potassium chromate necessary for sufficient inhibition is 500ppm under recirculating coolant and cavitation tests while 2, 000ppm under corrosion fatigue test.
2) Minimum level of emulsion oils necessary for sufficient inhibition is 1% by volume under all test conditions. However, some types of emulsion oils, accelerate corrosion of Cu-alloy.
3) If cooling water is contaminated by sea water, higher concentration of potassium chromate is required to inhibit corrosion, while specific type of emulsion oils is not affected up to 1% contamination.
4) From the standpoint of running cost and concentration control, potassium chromate may be more practical than the emulsion oils.

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An investigation to reduce the sensitivity to stress corrosion cracking of weld joints in high strength steels has been conducted, aiming to develop new method by which the hardened structure in heat-affected zone disappears, and a tempered-bead process and a flame heating process were deviced.
By these processes, the martensitic hardened structure in fusion lines of weld joints were changed into the softened structure of ferrite and spheroidal carbide, the so-called sorbitic structure, and, as the result, the sensitivity to stress corrosion cracking was found to be remarkably reduced.

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