Jean-Pierre Millet

Corrosion behaviour of zinc-nickel coatings, electrodeposited on steel

Abstract The electrochemical behaviour of sacrificial zinc–nickel coatings, electrodeposited onto low alloy steel is studied, in a neutral, saline, aqueous environment, in accordance with different preparation conditions (nature of the bath, density of electrodeposition current, degassing and chromate passivation). The stability of the sacrificial properties is determined by open circuit potential measurements over time, while the corrosion rate is evaluated using polarization resistance measurements and volt–amperometric diagrams. The effect of any porosity, revealing the substrate, is determined by Evans diagrams for various levels of simulated porosity. The corrosion behaviour of these deposits is closely linked to the nickel content and, to a lesser extent, to their structure. It is improved by a chromate passivation, but the chromating layer may deteriorate over time (long, continuous immersion). Over time, the corrosive products, based on zinc hydroxide, form a barrier film which slows down the dissolution of the material. After prolonged immersion, the sample surface is generally crazed due to the internal stresses present in the deposit.

Surface modification of martensitic stainless steels by laser marking and its consequences regarding corrosion resistance

Abstract Marking is of prime importance in the field of biomaterials to allow the identification of surgical tools or implants. Markings are obtained under industrial conditions with a pulsed Nd:YAG laser on a X30C13 martensitic stainless steel. Depending on laser parameters, two modes of marking are considered: a surface oxidisation mode and a matter photoablation mode. Electrochemical behaviour, i.e. passive state, corrosion rate, resistance to pitting, is evaluated in Ringer’s saline solution. Results are then explained on the basis of metallurgical modifications regarding the phases composition and the distribution of alloying elements. In oxidative regime, laser treatment drastically affects both microstructure and chemical composition. Specially, chromium depletion is measured in subsurface, leading to a loss of passive ability. In ablative mode the surface is transformed into a thin favourable layer of austenite. Moreover, the overall chromium distribution is little modified. Therefore, the passive character is maintained, but with nevertheless a shorter passive domain than base material one.

PVD aluminium alloy coatings: Environmentally friendly alternative to protect steel parts against corrosion

Abstract In the automotive and aeronautic industries, security is of prime importance and so corrosion prevention is essential. Steel parts were once protected with deposits such as cadmium or zinc - nickel produced using a chromatation treatment but recent environmental directives dictate that these methods are no longer acceptable. Aluminium sacrificial coatings have excellent corrosion behaviour but unfortunately they have poor mechanical characteristics and a fast dissolution rate. The present study investigated a possible solution: alloying the aluminium with a more noble element, chromium, in order to decrease the sacrificial galvanic effect of the deposit and therefore improve its lifetime. The corrosion protection afforded by such coatings in relation to their structure and mechanical properties was investigated. The coatings were produced on carbon steel by a vacuum PVD arc evaporation process at a pilot scale. Intrinsic electrochemical properties were determined on pure materials and on layers deposited on glass strip. Several elaboration configurations (pure as well as composite targets) were investigated. In most cases the coatings were stratified and composed of pure aluminium and numerous hardened AlxCry intermetallic phases. Chromium enrichment of aluminium based coatings induces not only a beneficial hardening effect on the surface characteristics (> 700 HV) but also significantly improves the corrosion behaviour of the coated pieces (increasing lifetime by up to three times compared to pure Aluminium).