PASSIVATION OF A ZINC NICKEL ELECTROLYTIC COATING - EFFECT OF CHLORIDES AND OF CARBON-DIOXIDE

The behaviour of steel sheets covered with Zn-Ni alloy (with 15% in ma ss of nickel) and immersed in solution under controlled atmospheres ha s been studied by chronopotentiometry. The electrolytes are aqueous so lutions with high or low in chloride concentrations, corresponding res pectively to salt spray test ([Cl-] # 0,51 mol.l(-1)) and rain water ( [CI-] # 0,3.10(-3) mol.l(-1)). Electrochemical measurements are perfor med under a gas flow with variable proportions of oxygen, nitrogen and carbon dioxide. The degradation occurring on these metallic electrode s is characterized by scanning electron microscopy. In the solution at high chloride content ([Cl-] # 0.51 mol.l(similar to 1))), the corros ion of the Zn-Ni coating is uniform, irrespective of the gaseous atmos phere considered. The high proportion of chloride ions insures the act ivation of the metallic surface. This alteration leads to the formatio n of zinc hydroxide when the gaseous phase permits the renewal of diss olved oxygen consumed by reduction on the alloy. This corrosion produc t gives rise to : - a structured homogeneous and passivating corrosion layer when the partial pressure of carbon dioxide in the gaseous phas e is about 300.10(-6) atm. This layer appears like a pilling up of cry stals in thin sheets. It is constituted mainly by zinc hydroxicarbonat e (Zn-5(OH)(6)(CO3)(2),nH(2)O) and / or hydroxichloride (Zn-5(OH)(8)Cl -2,nH(2)O). In a first step, in contact with the dissolved carbon diox ide, zinc hydroxide could transform in zinc hydroxicarbonate. In a sec ond step, this product could give zinc hydroxichloride by changing car bonates into chlorides; large packs of corrosion product forming a non homogeneous and not very adherent layer when the partial pressure of carbon dioxide in the gaseous phase is less than 10.10(-6) atm. In thi s case, zinc hydroxide gives zinc oxide by dehydration. This layer, wh ich is barely structured, has no protecting properties. In the solutio n corresponding to rain water with a lower chloride content (0.3 mmol. l(-1)), the degradation of the electrode keeps localized in the microf issures that are present on the coating at the initial state. They bec ome hollow and give an important cracking network which leads to local stripping of the steel substrate. So, the final development of red ru st pits is expected. This study shows that corrosion / passivation mec hanisms for Zn-Ni coatings are depending very strongly not only on the composition of the electrolyte, but also on the nature of the gazeous phase in equilibrium. The good behaviour of this type of coating when submitted to salt spray test is explained by the formation of a passi vating corrosion layer. This passivation is obtained in presence of hi gh chloride content and with a sufficient renewal of dissolved oxygen and carbon dioxide in the aqueous film at the metal surface.