Coprecipitation and Properties of Conversion Nano-layers Formed on Steel Plates during Iron Phosphating with Molybdate and Magnesium Nitrate Additives

Abstract Thin surface conversion layers formed by precipitation from slightly acidic iron phosphating solutions with different additives were examined by scanning electron microscopy (SEM) energy-dispersive x ray (EDS) and glow discharge optical emission spectroscopic (GD OES) surface analytical techniques revealing important details on their in depth elementary composition on the nanoscale. Some stoichiometric mineralogical analogies were also sought and explored based on the detected components of the coprecipitated nanolayers. During the spray phosphating process the molybdate additive formed codeposits through the whole insoluble iron phosphate-oxide-hydroxide type surface film that could incorporate traces of alkali and alkaline earth cations like Na, K, Ca, and Mg as well. Magnesium as an additive was then also added to the iron phosphating solution as its nitrate salt in different concentrations and its advantageous properties were evaluated after checking the corrosion resistance of the then e-coated (KTL epoxy painted) phosphate steel plates (Q-panels) by the standard salt spray test (ISO 9227), showing acceptable data up to 720 h exposure in 5 wt.% NaCl solution at 35 °C.