P.R. Seré

Atmospheric corrosion of painted galvanized and 55 % Al-Zn steel sheets: results of 12 years of exposure

Zinc or 55%Al-Zn alloy-coated steel sheets, either bare or covered by different painting systems, have been exposed for 12 years to the action of the urban atmosphere at the CIDEPINT station located in La Plata (34° 50′ South, 57° 53′, West), province of Buenos Aires, Argentina. The samples exposed surface was evaluated through periodical visual inspections, standardized adhesion tests, and electrochemical impedance measurements. The ambient variables monitored were average annual rains and temperatures, time of wetness, sulphur and chloride concentration, relative humidity, and speed and direction of the winds. It was found that in this atmosphere, the corrosion resistance of the bare 55% Al-Zn/steel sheets was higher than of the galvanized steel, and the polyurethane painting system was more protective than the alkyd and epoxy ones, which degraded after 6-7 years of exposure.

Morphology and Texture of Zinc Deposits Formed at the Edge of a Rotating Washer Electrode

In this paper, a rotating washer electrode was used to simulate the industrial conditions of strip edges during electrogalvanizing. Using this experimental setup, morphology and texture of the zinc deposits and dendrites formed at the edge of the electrode were studied. Dendrite precursors in the corner of the washer edge were well characterized and their nucleation and growth were also studied. The results indicated that the rotation speed has little effect on texture while, in contrast, current density modifies dendrite’s texture. It was found that dendrites are formed by platelets which are stacked on one another, which grow up in a series of steps oriented in the direction of growth of the dendrite’s stem yielding highly oriented structure. The presence of sodium ions in the electrolyte changes the morphology and shape of dendrites leading to rounded, ‘cabbage’-shaped crystals, while thiourea changes the morphology of both the deposit in the flat portion of the washer and the dendrites through adsorption on the zinc surface.

Preparation and Characterization of Silanes Films to Protect Electrogalvanized Steel

Silanes are an interesting alternative to chromate-based surface treatments for temporary protection of electrogalvanized steel. In this work, the protective behavior of 3-mercaptopropyltrimethoxysilane (MTMO), 3-aminopropyltriethoxysilane (AMEO), or 3-glycidoxypropyltrimethoxysilane (GLYMO) films applied on electrogalvanized automotive quality steel sheets has been studied. The silane coating morphology, composition, and porosity were characterized by scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), x-ray fluorescence, immersion in copper sulfate, and cyclic voltammetry. The corrosion protection was evaluated by polarization curves, electrochemical noise measurements, electrochemical impedance spectrometry, and accelerated humidity chamber tests. The results showed that the silanes protect temporarily electrogalvanized steel from corrosion. MTMO forms a relatively thick and cracked film. AMEO and GLYMO films were so thin that they could not be observed by SEM but silicon was detected by EDS. MTMO provided good temporary protection, being an alternative to replace Cr(VI) as protector of electrogalvanized steel.

Study of Commercially Available Epoxy Protective Coatings by Using Non-Destructive Electrochemical Techniques

SAE 1020 steel panels coated with different thicknesses of either epoxybituminous or epoxy enamel films of commercial quality in a naturally aerated 3% NaCl solution were investigated. The progression of corrosion as a function of exposure time was studied with electrochemical impedance spectroscopy and corrosion potential measurements. Simultaneously, böth the salt spray cabinet test (ASTM Standard D 117/79) and the monitoring of the rusting grade (ASTM Standard D 610/85) were also performed. Based on the overall experimental results, the coatings have been ranked in terms of the effectiveness of their anticorrosive properties in a chloride containing electrolyte. Furthermore, a comparison of the visual assessment, water permeability, impedance and corrosion potential data indicates that the nondestructive and fairly straightforward electrochemical techniques are a very powerful tool for studying the highly complex metal/organic coating/aqueous electrolyte interface.