The distribution of pit depth on type 304 stainless steels generated in FeCl3 solution has been investigated as a function of immersion time, in view of demand for experimentaldata to establish a deterministic model for their life prediction. The early depth distribution showed several peaks corresponding to those in corrosion potential which suggests wavelike generation of pits. The distribution in growing stage split at approximately 40μm which seems to be the critical depth for maturation. The matured pit can continue to grow as long as the corrosion potential exceeds repassivation potential. The profile of the whole distribution became less steep with time. Although the rate constant for increase of pit depth did not seem to be uniform, the maximum pit depth was analyzed to study the kinetics of pit growth. The depth (h) was proportional to the square root of time: h=kt1/2 The growth rate was independent of potential and agitation. A rise in temperature caused increase in both pit density and growth rate.
Corrosion weight loss tests have been carried out of the Al (110) plane in the 3N HCl solutions added each of CH2=CHCOOH, HCOOH, and MeCOOH at 50°C, 60°C, and 70°C. The adsorption energies ΔH (kJ/mol) of each of the three carboxylic acids were obtained by the correlation between log K and 1/T. The ΔH values obtained are 137.86 (CH2 CHCOOH)>66.98 (HCOOH)>27.74 (MeCOOH) and these velues have a linear correlation with the values of Δico of the acids. The Δico values have a close correlation with the corrosion inhibition of the carboxylic acids due to the adsorption of the acids on the Al single planes. The order of corrosion inhibition obtained by the polarization curves for the acids is CH2=CHCOOH>HCOOH>MeCOOH and the ΔH values obtained for the three carboxylic acids support the close correlation between adsorption and corrosion inhibition of the carboxylic acids on the Al (110) plane.
The photoelectrochemical behavior of Cu coated with TiO2 by sol-gel method was studied, aiming at the cathodic protection of Cu by the TiO2 coating under illumination. It was found that the heat treatment in the coating process played a crucial role in determining the photo-effect of TiO2 coating on the Cu substrate, Heat treatment at temperatures above 400°C was found to be essential to cause the potential of the TiO2 coated Cu specimen to shift toward less noble direction under illumination, The optimum heat treatment temperature for catholic protection was found to lie in the temperature range between 600°C and 700°C, in which the specimen exhibited the least noble photopotential. The increase in coating thickness more than 0.1μm seemed to have little influence on the photopotential in deaerated solutions. Under the appropriate heat treatment conditions, the results demonstrated an effective catholic protection for Cu by the TiO2 coating over the tested solution pH range from 4 to 12, because the photopotentials were in the immunity domain in the E-pH diagram of Cu.
Corrosive gas such as H2S and CO2 makes for very severe environments for steel. It is well known that Hydrogen Induced Cracking (HIC) and Sulfide Stress Corrosion Cracking (SSCC) are major problems which occur under these corrosive environments. We have investigated the corrosion and cracking behavior of coated steel under a H2S and a H2S/CO2 environment. Panels of high tensile strength steel were U-bended in order to impart residual stress. After surface treatments, several epoxy coatings were applied for 50 to 400μm, then cured. Corrosion tests were conducted in autoclaves under increased pressure and the corrosive gas environments (H2S, H2S/CO2). The following results were obtained: 1. Corrosion rate of the steel panels increased parabolically. 2. Film appearance and adhesion to the substrate did not change if corrosion loss under the film has not reached 5mg/cm2. 3. For samples with thin coating thickness (e.g. 50μm), underfilm corrosion quickly propagated, blistering and peeling were observed, consequently HIC or SSCC was induced. 4. For samples with thick coating thickness (e.g. 200μm), permeation of the corrosive gas proceeded very slowly, and rates of the underfilm corrosion were considerably slow. In such cases, FeS layers were generated at film/steel interface. The FeS layers were fine enough to prevent further corrosion.
The mechanism of the interfacial corrosion of painted steel sheets and the role of phosphate conversion and zinc coatings on paint performance were reviewed. XPS and EPMA analyses were successfully used to anaylyse the failured interfaces. The paint delamination process changes depending on the pretreatment and corrosion conditions. In the wet atmosphere such as salt spray test (SST), the delamination of the phosphated steel occurrs cathodically by corrosion-induced alkalline attack on the resins and phosphate films. Accordingly, the alkalline resistance of the phosphate films is of prime importance for improving paint performance. On the galvanized steel, however, anodic dissolution of zinc accelerates the paint creep back by forming a new corrosion cell between one anode and two cathodes which are localized at the scribe and the leading edge of delamination, respectively. On the other hand, in the cyclic corrosion test, anodic mining of galvanized steel is suppressed by a lack in water during drying and a reduction in potential difference of Zn/Fe due to higher temperature compared to the SST, resulting in a low corrosion rate. The effect of the minor elements such as Al and Pb present in zinc layer on the bond durability of the epoxy-bonded galvanized steel has been also discussed.