Abstract
Technological greases based on polyphosphates of alkali metals have great prospects for application at high-temperature machining of steel. An important task is to study the anti-corrosive effect of polyphosphates on steel at elevated temperatures. Temperature ranges, in which phase transformations of metaphosphate and sodium tripolyphosphate, as well as interaction with iron oxide, occur, were established using a thermogravimetric method. Composition of products of interaction between metaphosphate and sodium tripolyphosphate and scale is determined employing an X-ray phase analysis. It was established that in the region of temperatures of hot steel deformation the iron oxides, contained in scale, are dissolved in molten metaphosphate and sodium tripolyphosphate. As a result of interaction between sodium metaphosphate and iron oxide, the mixed polyphosphates Na 3 Fe 2 (PO 4 ) 3 and Na 9 Fe 2 (P 3 O 10 ) 3 are formed. It is shown that sodium tripolyphosphate almost does not participate in the interaction with the iron oxide of scale. Comparison of the results of corrosion test of the steel surface, treated in the presence of a polyphosphate lubrication and sodium chloride, testifies to the high anti-corrosive effect of polyphosphates. Thus, the time before the emergence of first signs of corrosion in the presence of polyphosphates increased fourfold, while the degree of corrosion damage was reduced by 40 times. It was established that at the deformation treatment of steel at a temperature of 800 °C in the presence of a polyphosphate lubricant, corrosion resistance is due to the formation of a barrier film at the steel surface, consisting of mixed polymer phosphates.
Highlights
Conversion treatment of steel products is a common type of surface treatment
This is predetermined by the need to protect corrosive-active steel surface against aggressive effect of the environment
Polyphosphates act as an inorganic grease for the steel surface treated at elevated temperature
Summary
Conversion treatment of steel products is a common type of surface treatment This is predetermined by the need to protect corrosive-active steel surface against aggressive effect of the environment. Phosphate coating is typically applied as a conversion coating Such a coating can act as the adhesion layer for paint and varnish treatment, oil-retaining or a separate protective film. One of the directions of active use of phosphate blends in modern manufacturing is the modification of tribological characteristics of the surface of steel products. In this case, polyphosphates act as an inorganic grease for the steel surface treated at elevated temperature. There is a relevant, technologically and economically motivated, task on the optimization of anti-frictional polyphosphate compositions that would ensure high anti-corrosion properties of the steel surface under conditions of high-temperature treatment
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