A Study of Coating Process of Cast Iron Blackening

An improved chemical composition for the production of the black oxide coatings, termedblackening on cast Iron, and its particular relation to optimum conditions for cast Ironblackening in the presence of chromium and silicon is studied. Black oxide coating processbath and different preparating conditions of substrates were investigated and their effectson coating quality were discussed. Comprehensive researches illustrate that at higherchemical bath temperatures and immersion time of parts, the coating thickness increases up to one micrometer, but protecting effects and adhesion of coating decreases. Chemicalbath temperature between 130C to 140C and contact times of 20 minutes was observed as optimum. In addition, the effects of the compositions of the bath on the quality of blackcolored layer are discussed. It is shown that when potassium salts were used in blackening bath the quality of coating improved and optimum compositions of chemical blackeningbath is given. The optimum bath compositions were obtained as follows: 70 weight percent of potassium hydroxide, 10 weight percent of potassium nitrite and 20 weight percent ofpotassium nitrate. Furthermore; it is shown that acid pickling with one molar sulphuric acid for five minutes before blackening step result in higher adhesion and coating quality.