The effect of processing variables on steel surface chemistry

Differences in processing at all stages in the steel making process can effect the surface chemistry of steel. The surface chemistry can dictate the performance of many different products and processes including electrogalvanized steel. If the steel chemistry is not controlled sufficiently, the zinc electrodeposit will be subject to hydrogen blistering.;This dissertation examined the effect of many processing variables on both the organic and inorganic contaminants on steel surfaces. Processes both before the electrogalvanizing line and those after the electrogalvanizing line were examined. Mechanisms were determined for reactions of organic oils with the steel surface. Reaction between the carbonyl functionality, present as an acid or an ester, and iron hydroxide to form a tenaciously adherent iron soap was demonstrated using infrared spectroscopy and mass spectrometry.;Thermodynamics and kinetics of segregation of metallic elements to the surface of the steel were determined. It was shown that the primary mechanism for segregation of elements such as aluminum, silicon, chromium, and titanium is oxidation during the anneal. Analysis of the kinetics indicated the feasibility of this mechanism.;The mechanisms of electrocleaning were elucidated, based on both electrochemical and physical effects. Improved methods of electrocleaning were described based upon optimization of the cleaning frequency. It was found that increasing the frequency of anodic and cathodic cycling increases the cleaning efficiency up to about 5 Hz. Above this frequency bubbles, which provide physical scrubbing and convection near the interface, can not form rapidly enough to be effective. Increased current density was shown to be effective in improving electrocleaning.;Two evaluative techniques were developed to assist in the evaluation of the steel surface chemistry. The blister backcharging test entails electrochemically forcing atomic hydrogen through a steel membrane until it reaches the interface with an impermeable coating, such as zinc. The pressure of the hydrogen at the interface forces the coating to blister. The blister density is a measure of the interfacial adhesion strength. The second technique developed was based upon the use of cyclic voltammetry. It was demonstrated that the slope of the hydrogen evolution region of the voltammogram is inversely proportional to the amount of carbonaceous material on the steel surface as measured by x-ray photoelectron spectroscopy.