A.R. Di Sarli

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 Experimental data withdrawn from salt spray cabinet tests performed for 261 days and adhesion measurements are also discussed.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.

The use of electrochemical impedance measurements to assess the performance of organic coating systems on naval steel

Abstract Electrochemical impedance and corrosion potential measurements were carried out on naval steel coated with various organic paints systems whilst immersed in 3% NaCl solution. These techniques have been used to determine the mechanisms involved in the corrosion process as well as to assess the protective power of paint systems commonly used in marine environments. The paint systems studied consisted of a zinc-rich ethyl silicate primer (ZRP) with a chlorinated rubber or epoxy poly(amide) topcoat, in different thicknesses. Prior to immersion the painted specimens were subjected to thickness and adhesion measurements. After immersion, impedance measurements were carried out periodically as well as the standard tests according to ASTM D-610/68 and ASTM D-714/87. The latter were undertaken in order to assess the degree of rusting and blistering of the painted steel surfaces, respectively, in an attempt to correlate the visual observations with the electrochemical data. Complex plane plots of the resistive and reactive components of impedance, obtained over the frequency range from 65 KHz to 1 mHz were analyzed in accordance with resistive-capacitive equivalent electrical circuit models assumed to describe the painted metal/solution interfaces. The experimental results suggest that both the type and the thickness of the topcoat paints could be of paramount importance in controlling the rate of action of the zinc-rich primer and hence allowing an extension of its useful life. At the same time, the cathodic protection increases the tendency of cathodic reactions occurring at the coated steel interface and consequently increases the likelihood of delamination and blistering of the coating near a defect.

A mathematical basis for calculating the water permeability of organic films supported by metal substrates

Abstract A mathematical method aimed at establishing the water permeability of organic coatings applied on metal substrates is described. The equations obtained were derived from Picks second law. From a knowledge that the dielectric constant of a membrane varies with the content of absorbed water, the permeability may be calculated using values of the dielectric capacitance obtained from a measurement of the impedance vector as a function of the immersion time at a fixed excitation frequence. A calculus algorithm is presented and a discussion made of the details which must be taken into account to guarantee reliable results and to explore the possibilities of the technique.

Evaluation of steel/primer based on chestnut tannin/paint film systems by EIS

The influence of a tannin pretreatment applied to slightly rusted steel panels on the anticorrosive performance of alkyd paint systems has been investigated using electrochemical impedance spectroscopy (EIS) and open circuit potential measurements. From EIS and corrosion potential data analysis it was found that (1) there is not a significant influence of the tannin pretreatment on the anticorrosive protection level, which depends particularly on the barrier effect afforded by the applied paint system (however, as the corrosion process starts, tannins may cause the repassivation of steel); (2) the enhancement of the protection level afforded by the tannin primer was restricted to short immersion periods and corrosive media that were not very aggressive; and (3) water uptake was not significantly affected by the presence of the tannin primer.

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.

Study of the pigment type effect on the corrosion behaviour of epoxy painted steel/ sea water systems

The effect of different kind of pigments on the corrosion resistance properties of an epoxy paint has been investigated using electrochemical impedance spectroscopy and open circuit potential measurements. Painted naval steel samples were studied during the exposure to artificial sea water. The epoxy paint coatings were prepared employing red lead, zinc, red iron oxide or titanium dioxide as pigment with 0.8 of PVC/CPVC ratio. From the EIS and corrosion potential data analyis was found that the anticorrosive protection given by an anticorrosive (pigmented with red lead) and a top coat (pigmented with red iron oxide) epoxy painting scheme was both more lasting and more effective. This was attributed to the fact that both the effective adhesion provided by the chemical bonding between the steel surface and the epoxy paint groups and the constant high barrier effect afforded by this type of top coat were a perfect complement for the high corrosion inhibitive capacity of the red lead pigment.