Cecilia Inés Elsner

Comparative corrosion behaviour of 55Aluminium-Zinc alloy and Zinc hot-dip coatings deposited on low carbon steel substrates

Abstract A comparative study of the anticorrosive behaviour of 55Aluminium–Zinc-alloy and Zinc coatings, both applied by the hot-dip process to steel, was performed using salt spray and humidity cabinet tests as well as immersion ones. Surface degradation of the coated steel was studied by SEM and EDS, whilst the corrosion products were characterised using XRD. The experimental results showed that the corrosion resistance of the 55Al–Zn alloy coating was higher than that of Zn alone particularly in presence of chloride ions. The morphological characteristics of the corrosion products formed also differed.

Evaluation of the surface treatment effect on the anticorrosive performance of paint systems on steel

Abstract Electrochemical impedance spectroscopy studies were carried out for paint coated carbon steel sheets under free corrosion conditions when exposed to 3.5% NaCl solution for 405 days. Five anticorrosive primers [epoxy-polyamide/red lead/ferric oxide; alkyd/red lead; epoxy/zinc–aluminium; zinc-rich ethyl-silicate and epoxy/zinc phosphate] were used while changing the steel surface treatment condition. The EIS data was interpreted and discussed in terms of the time dependence of the electrical (paint coating) and electrochemical (steel substrate) parameters associated to interfacial processes describing the metal/paint system deterioration. Furthermore, the progress of the coating delamination process was estimated by the empirical expression relating the measured/specific double layer capacitance ratio. Experimental data withdrawn from salt spray cabinet tests performed for 261 days and adhesion measurements are also discussed. On the basis of the electrochemical, visual inspection and standardized tests results it was concluded that the best corrosion performance was provided by the epoxy-polyamide/red lead/ferric oxide anticorrosive paint without any marked effect of the surface treatment; followed by those anticorrosive systems including dry or wet sandblasting in presence of inhibitor. This result has been attributed to the fact that such surface treatments can improve the barrier properties and/or the inhibiting corrosion protection and the steel/paint adhesion properties or reduce the osmotic pressure effect.

The surface condition effect on adhesion and corrosion resistance of carbon steel/chlorinated rubber/artificial sea water systems

Abstract The influence of both substrate structure and roughness on the adhesion strength and corrosion resistance of carbon steel/chlorinated rubber varnish/artificial sea water systems was analysed. Experimental results obtained from electrochemical and normalized tests showed that the metal/coating adhesion increases before immersion as does the steel roughness. This is due to the enlargement of the number of active sites for metal/coating bonding: not only the specific characteristics of such a system but also those arising from the environment must be taken into account. A high dry adhesion grade does not always lead to a good practical performance, although the substrate structure and roughness changes did not produce significant changes in adhesion strength and corrosion resistance. The martensitic structure showed the worst performance in all the tests.

The influence of the method of application of the paint on the corrosion of the substrate as assessed by ASTM and electrochemical method

ConclusionsAC together with DC and standardized procedures have considerable value in assessing the protective ability of a paint film by scientific studies of anticorrosion problems. Knowledge about the changes in permeability, conductivity and adhesion properties of coatings when exposed to aggressive environments is important, not only because it indicates how the coating may behave in practice, but also because it gives an insight into its protective properties in prevention of corrosion of the metal substrate.From the values of corrosion potential, ionic and charge transfer resistance with immersion time, the alkyd paint applied by means of four different methods showed relatively good protective properties in the order Sy>R ≅ Sn but very poor ones for B panel. On the other hand, when submitted to the salt spray cabinet test, all the steel panels coated with unscribed alkyd films have shown high resistance to the strongly aggressive medium. Likewise, the cross scribed panels show only corrosion at and close to the cross-scribing.Taking into account the overall results, the most effective application method was spraying followed by roller coating, spinning and lastly brushing. Further work needs to be done to confirm some of the assumptions made here.

Effect of the plastic deformation on the electrochemical behavior of metal coated steel sheets

Abstract Protective metallic coatings based on Zn or Al–Zn alloys are usually used to protect steel against corrosion. When coated steel sheets are subjected to formed processes, its corrosion behavior is affected due not only to changes of the galvanic coating texture but also of the twins and cracks developed. The aim of the present work was to evaluate the performance of galvanized steel or steel/55%Al–Zn sheets mechanically deformed and exposed to 0.5 M NaCl; 0.5 M NaClO 4 and 0.5 M Na 2 SO 4 solutions. Samples built up from commercially available coated steel sheets were uniaxially deformed up to reach an elongation of 1.3, 6.25 and 12.5%. The corrosion behavior of deformed and non-deformed samples was evaluated by means of DC electrochemical techniques. The experimental results allowed to infer that: (1) there were evident differences in the kinetics of the corrosion reactions taking place at the surface of both metallic coatings and (b) such kinetics is a function of the deformation degree as well as of the electrolyte aggressiveness.

Effect of zinc crystals size on galvanized steel deformation and electrochemical behavior

Hot-dip galvanized steel sheets with different spangle sizes were deformed by means of rolling and tension. The change of preferential crystallographic orientation and of superficial characteristics due to the deformation was analyzed by means of both X-rays diffraction and optical and scanning electronic microscopy. A correlation between such changes and the involving deformation modes was intended to be done and the spangle size influence on these modes was studied. Coating reactivity change due to the deformation was investigated by means of quasi-steady DC electrochemical tests. The results allow to infer that, in great spangle samples, the main deformation mechanism is twinning whereas in small spangle ones, pyramidal slip systems happen as well. The increase of the reactivity with the deformation is greater in tension than in rolling and it is more important in small than in great spangle samples.

Protection of galvanized steel with silanes: its comparison with Chromium(VI)

The corrosion behavior of hot dip galvanized steel (HDGS) pre-treated with mercaptopropyltrimethoxysilane and a commercial sulfur-bearing silane was studied. Electrochemical polarization, electrochemical impedance spectroscopy, and electrochemical noise tests showed that silane coatings have a corrosion protection performance similar to the usual hexavalent chromium HDGS passivation treatments. It is also evident that the silane films protect the zinc surface through the formation of an isolating barrier. Through voltamperometric studies it was possible to define an electrochemical porosity of the protective coatings. Based on copper sulfate tests and electrochemical porosity results the films protection capability was evaluated, showing that silane treatments have similar or even better protection performance than standard chromium passivation.

Electrochemical characterization of chromate free conversion coatings on electrogalvanized steel

The chromate conversion treatment is widely used, but it requires highly toxic chromic acid solutions with the consequent effluent disposal and ecological problems. The removal of these toxic chemicals is considered a priority within European Union. The corrosion resistance of three alternative treatments applied on electrogalvanised steel, and immersed in aerated 0.3 M Na2SO4 solution, pH 10, at 25 °C, was investigated using electrochemical techniques. Their performance was compared with the obtained using the traditional Cr6+-based treatment in the same conditions. The achieved results show that the alternative coatings exhibited discrete protective properties in the sulphate solution. The nitro-cobalt chemical conversion treatment showed similar protective properties than the traditional Cr6+-based treatment, while with the Cr3+-based treatment those were very poor. The phosphate treatment initially performed acceptably but as the time elapsed, its protective properties decreased.

Microstructural, mechanical and electrochemical characterisation of biomaterial ASTM F745 cast by vacuum

Abstract Countergravity low pressure casting (CLA) was performed to enhance the properties of ASTM F745 stainless steel (SS), which is usually used as biomaterial. The macro- and microstructures were compared with those obtained by the conventional process of investment casting (IC). The SS cast by CLA (SSCLA) exhibited a smaller size of solidification cell and finer dendritic microstructure. The average of its dendritic primary spacing was 110·4 μm, while for the same steel cast by IC (SSIC), it was 186·7 μm. The density of non-metallic inclusions δI in the SSCLA was 717 I mm−2, being the majority of them smaller than 1·5 μm. In the case of SSIC, δI was 852 I mm−2, with a size distribution of up to 8 μm. The SSCLA showed a higher breakdown potential than the SSIC, the values being 0·300 and 0·210 V(saturated calomel electrode) respectively, which means a higher resistance to suffer localised corrosion. Finely, the CLA process also allowed obtaining better mechanical properties.

Mechanical, electrochemical and magnetic behaviour of duplex stainless steel for biomedical applications

Mechanical, electrochemical and magnetic properties of duplex stainless steel were analysed to evaluate its use as biomaterial, comparing the results with those obtained for austenitic stainless steel. Yield and ultimate tensile strengths are almost twice in duplex stainless steel, being the values 870 MPa and 564 MPa, respectively. The electrochemical test revealed that this material has lower susceptibility to localised corrosion because of its greater passive range, 1 V from the open circuit potential, while the austenitic stainless steel exhibited a passive region of 0.370 V. Both steels behave as soft magnetic materials, however, duplex stainless steel has higher magnetic saturation and remanence, while austenitic stainless steel is more prone to heating when exposed to a magnetic field.