Abstract
The hypothesis on the matrix (composite) structure of the iron (III) tripolyphosphate precipitate and polyphosphate coatings on the steel substrate has been justified experimentally. According to the hypothesis, that iron (III) tripolyphosphate acts as a matrix forming agent while water-soluble sodium tripolyphosphate corrosion inhibitor is a filler. Be means of the chemical synthesis method, it has been demonstrated that the chemical basis of the coating formed on the steel surface in the STPP solution is iron (III) tripolyphosphate. It has been established that the composite structure of tripolyphosphate coatings and precipitates is determined by the nature of iron (III) tripolyphosphate. The conducted investigation on the composition of the iron (III) tripolyphosphate precipitate and tripolyphosphate coating on steel, using the gravimetric method with washing have proved the presence of STPP. The results of X-ray spectral analysis have revealed that STPP in the structure of composite coating is situated not only as a filler inside the matrix but also as an additional layer on its surface. The chemical protection mechanism of the tripolyphosphate coating on steel in atmospheric conditions has been justified. The acquired knowledge would expand the theoretical and practical concept of the structure and properties of tripolyphosphate coatings on the steel substrate and their protection mechanism in conditions of atmospheric corrosion. The application of new knowledge in researches aimed at the development of ecologically safe methods of tripolyphosphate coating deposition with estimated complex of properties, would increase their efficiency.
Highlights
The phosphating process is widely used in industry for steelwares because of high performance, low cost and ease of deposition of phosphate coatings.Owing to multifunctionality and a wide range of practical applications of phosphate conversion coatings, the requirements to their quality and deposition techniques, including economical and ecological aspects, have increased significantly in recent years.One of the priority directions of improving the phosphating process is the reduction of the ecological hazard of technological processes [1].An example of such project is the replacement of traditional acidic phosphating solutions with aqueous solutions of polyphosphates.Technology of organic and inorganic substancesAmong the most commonly used polyphosphates, a particular place is taken by sodium tripolyphosphate (STPP)
Upon mixing the solutions of STPP and FeCl3, the iron (III) tripolyphosphate precipitate forms, which has a white color at a synthesis temperature of 20 °С, and light-yellow color at a a synthesis temperature of 80 °С
By means of the chemical synthesis method, it has been demonstrated that the chemical basis of the coating formed on steel in the aqueous STPP solution is iron (III) tripolyphosphate
Summary
The phosphating process is widely used in industry for steelwares because of high performance, low cost and ease of deposition of phosphate coatings.Owing to multifunctionality and a wide range of practical applications of phosphate conversion coatings, the requirements to their quality and deposition techniques, including economical and ecological aspects, have increased significantly in recent years.One of the priority directions of improving the phosphating process is the reduction of the ecological hazard of technological processes [1].An example of such project is the replacement of traditional acidic phosphating solutions with aqueous solutions of polyphosphates.Technology of organic and inorganic substancesAmong the most commonly used polyphosphates, a particular place is taken by sodium tripolyphosphate (STPP). One of the priority directions of improving the phosphating process is the reduction of the ecological hazard of technological processes [1]. An example of such project is the replacement of traditional acidic phosphating solutions with aqueous solutions of polyphosphates. An aqueous STPP solution is basic electrolyte of phosphate nature, stable in time and non-volatile at elevated temperature. This allows for it to be considered promising for the development of a more ecologically safe technology for deposition of phosphate-based coatings
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