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The overaging treatment of a continuous annealing line was simulated in the laboratory. The chemical state of the surface of low carbon steel sheet was determined by using ESCA and SEM. The surfaces of a conventional, low alloyed deep drawing quality (DDQ) steel and a dual phase (DP) steel alloyed with Mn and Si were characterised in simulated and production line conditions. The influence of the surface state of the steels on the formation of surface phosphates was investigated. During overaging in the temperature range 200–400° C in a protective H2 + N2 atmosphere iron oxide is reduced to metallic iron. The alloying elements manganese and silicon are oxidised. Silicon especially is markedly enriched on the surface, in the form of pure Si02, due to its high diffusibility. The manganese and silicon oxides form particles on the surface during the overaging treatment. The results indicate that iron oxide is dissolved in the particles. Sodium and calcium are also present in their oxide states on the surfaces of the production line sheets; they originate in the rinsing waters used. Temper rolling reduces the amount of surface oxide particles to about half the prerolling value. It is suggested that this is due to the stripping of particles from the surface and the pressing of particles into the steel matrix. The phosphating experiments show that the steel sheets treated in the simulation furnace and taken from the production line form surface phosphates readily. The phosphate crystals are small, uniform, and cover the entire surface.

Electrodegreasing of cold rolled steel sheets in an alkaline solution containing silicate was investigated by surface sensitive techniques. The influence of the contamination levels of the sheets and the solutions was studied. A model for the electrodegreasing mechanism is given: desorption of hydrocarbons from the surface occurs at polarization of the surface; readsorption is prevented by formation of microemulsions in the solution. Anodic electrocleaning gives a more efficient cleaning than cathodic degreasing. It is suggested that the number of bonds is reduced and the desorption from the surface is facilitated by oxidation of double bonds in the hydrocarbons adsorbed on the surface of the anode. Sodium silicate, Na2SiO2(OH)2, is deposited on the surface during electrodegreasing. More silicate is found after cathodic polarization than after anodic polarization. The precipitation at the cathode is explained by an increase in the pH value at the surface.

A method for corrosion testing of phosphatized steel samples has been described. The specimens, treated with different phosphatizinp solutions and covered by a chemically inert paint film, have been exposed in a sulphur-dioxide-polluted atmosphere with 90% relative humidity at 22.0"C. The corrosion rate has been measured as an increase in weight. The corrosion resistance shows a good correlation with the crystal coverage of the zinc phosphate layers. Filiform corrosion can be inhibited by zinc phosphate as well as iron phosphate coatings. However, underfilm corrosion is more frequent on the iron phosphatized samples.Beschrieben wird eine Methode zur Priifung von phosphatiertem Stahl. Dabei werden die mit verschiedenen Phosphatierrnitteln behandelten Proben, die mit einem chemisch inerten Anstrichfilm beschichtet sind, bei 22,O "C einer mit Schwefeldioxid verunreinigten Atmosphare bei 90% relativer Feuchtigkeit ausgesetzt. Die Korrosionsgeschwindigkeit wird aus der Gewichtszunahme bestimmt. Die Korrosionsbestandigkeit ist gut korrelierbar mit dem Kristallbedeckungsgrad der Zinkphosphatschichten. Fadenformige Korrosion kann sowohl durch Zinkphosphat-als auch durch Eisenphosphatschichten gehemmt werden, doch ist diese Unterschichtkorrosion bei den mit Eisenphosphat bedeckten Proben haufiger anzutreffen.