Degradation and crystalline reorganization of hureaulite crystals during the manganese phosphating of a high strength steel

This work shows that the manganese phosphating of steel is a cyclic process where crystalline reorganization is a critical stage in the development of phosphate conversion coatings. It is shown that oscillations in the manganese rich hureaulite crystal size are produced during the crystalline reorganization stage, which is related to the changes in the coating weight reported previously by various authors due to the degradation-dissolution of the crystalline layer of manganese rich hureaulite. The presence of defects (pits) in the prismatic crystals promotes their degradation due to an acidification process by differential aeration. This localized acidification process produces a composition change in the prismatic crystals that triggers a crystalline reorganization of the coating which results in a change of morphology of the hureaulite which, in turn, leads to the formation of an iron rich hureaulite inner layer. The parabolic growth of the coating may be due to the formation of this layer due to a localized attack on the steel.