Petrus Christiaan Pistorius

The nucleation and growth of corrosion pits on stainless steel

The nucleation and growth of corrosion pits on stainless steels in chloride is described. Pits arise from distinct sites on the surface that are destroyed after reaction. Metastable and stable pits grow at a diffusion-controlled rate. In the metastable state metal dissolution occurs through a perforated cover: the pit only achieves stability when this cover is no longer necessary to maintain the diffusion barrier. The growth of the pit and the transition from metastability to stability is described by the pit stability product. Recent results show that nucleation of the corrosion pit occurs by a microscopically violent event, observed as a sharp tiny current transient which initiates metastable pit growth. Many such nucleation events do not achieve metastability, however, and die immediately after nucleation.

Growth of corrosion pits on stainless steel in chloride solution containing dilute sulphate

The effects of dilute sulphate on metastable and stable pitting of 304 stainless steel in chloride solution have been studied. The presence of sulphate causes the distribution of available pit sites to be shifted to a higher potential, implying that pit nucleation is inhibited. Pit propagation, in both the metastable and stable states, is also inhibited by the sulphate ion. The reduced pit propagation current densities are described quantitatively with respect to the effect of sulphate on the solubility of the metal cation in the pit anolyte. The results are consistent with the observation that metastable and stable pits grow under diffusion control, at a rate which is independent of electrode potential. Pit nucleation and propagation in stainless steel are two distinct processes, of which only the former is directly affected by the potential.

Aspects of the effects of electrolyte composition on the occurrence of metastable pitting on stainless steel

The rate of formation of metastable pits on type 304 stainless steel surfaces in chloride solution is examined as a function of the chloride concentration and pH. The frequency of pitting events decreases as the chloride concentration is reduced; the effect arises from a concomitant reduction in the number of available sites at which metastable pitting can take place. The electrolyte pH has no observable effect on the frequency of metastable pitting, and no effect on the number of available sites. The results are consistent with the hypothesis that the ability of a metastable pit to develop at a particular site depends on the geometry of the site. Preliminary experiments on the effects of aeration show that dissolved air reduces the frequency of occurrence of metastable pitting current transients. The effect of oxygen remains unexplained.

Surface Roughness and the Metastable Pitting of Stainless Steel in Chloride Solutions

Abstract The effects of surface roughness on metastable pitting susceptibility of type 304 (UNS S30400) stainless steel (SS) in chloride (Cl−) solution and the transition to stable pitting corrosion were investigated. A smoother surface finish reduced the frequency of metastable pitting because the number of metastable pit sites available was reduced. Once formed, however, a metastable pit had a greater probability of achieving stable pit growth if the surface was smoother. Observations were rationalized qualitatively in terms of the geometry of the available metastable pit sites and their ability to establish a diffusion-controlled dissolution rate.

Calcium Modification of Spinel Inclusions in Aluminum-Killed Steel: Reaction Steps

Calcium treatment is a well-established way to modify solid alumina inclusions to liquid or partially liquid calcium aluminates. Spinels (Al2O3·xMgO) can also form in liquid steel after aluminum deoxidation. Like alumina, the spinels can be modified readily to liquid inclusions by a calcium treatment. The modification of spinels was studied by observing the transient evolution of inclusions, in laboratory and industrial heats. Spinel modification involves the preferential reduction of MgO from the spinel, with Mg dissolving in the steel, and it proceeds through transient calcium sulfide formation, just like in the case of alumina inclusions. Because magnesium dissolves in steel after the calcium treatment of spinels, the reoxidation of the melt will produce new spinels.

Model predictive control of an electric arc furnace off-gas process

Abstract This paper shows that an electric arc furnace off-gas system can provide valuable manipulated variables for feedback control which can improve furnace efficiency and contribute to safety in the workplace. Model predictive control (MPC) is used to illustrate this concept using practically motivated control objectives. An initial verification of a non-linear furnace model with plant data is shown. The design of MPC controllers for the furnace is discussed and results are shown by way of simulation. Evaluation of the final controller against traditional manual operation is done, and the setpoint tracking capability of the controller is tested.

Rate-determining steps for reduction in magnetite-coal pellets

A comprehensive modelling approach and experimental results were used to test possible rate-determining steps for direct reduction in pellets containing fine magnetite and coal. In support of the modelling approach, a survey of rate constants from the literature is presented. Heat transfer was found not to be limiting for the reduction reaction (although conduction within the pellet limits the rate of devolatilisation of the coal). For average rate constants, the chemical reactions are under mixed control (by the reduction and Boudouard reactions). The reductant also affects the product quality, since most of the sulphur in the reductant is retained in the direct-reduced product.

Design Aspects of Electrochemical Noise Measurements for Uncoated Metals: Electrode Size and Sampling Rate

Abstract Observed and simulated electrochemical noise (EN) from general and localized corrosion on uncoated metals was used to test theoretical concepts regarding the relation of electrode area and...

Minimisation of calcium additions to low carbon steel grades

Abstract This investigation aimed to determine the in plant feasibility of decreasing, to an amount close to the minimum literature value, the calcium addition to liquid steel for prevention of alumina buildup during continuous casting. Six plant trials were carried out at calcium additions of 0·14 kg/t of steel (reduced from the original 0·19 kg/t), added to the second ladle of a two or three ladle sequence. Total oxygen samples were taken at the ladle furnace and tundish to determine total oxygen and nitrogen contents of the steel. The total oxygen content at the ladle furnace varied between 19 and 26 ppm, with a slight degree of reoxidation between the ladle furnace and the caster. Alumina clogging was successfully prevented by the addition of 0·14 kg calcium/t of steel during the first five trials. During the sixth trial the submerged entry nozzle (SEN) failed and, although the stopper behaved as if clogging occurred, this behaviour was caused by the poor perfomance of the SEN rather than actual clogging. Microanalysis of inclusions in steel samples revealed a distribution in degrees of modification between different inclusions, and the formation of a substantial amount of CaS (which is taken to indicate overmodification, based on equilibrium calculations). However, the CaS is mostly associated with at least partly liquefied oxide inclusions, which is likely to reduce the potential clogging effect of solid CaS.

Economic evaluation and design of an electric arc furnace controller based on economic objectives

Controllers are often designed to satisfy functional objectives, without a thorough analysis of the resulting economic impact. A model predictive controller, designed based on economic objectives, is used to control electric arc furnace variables that are usually under manual control. To make the controller simulation as realistic as possible, the economic data used are taken from an operating furnace, and measurements are only fed back as and when they typically become available. The economic benefits of model predictive control over manual control are also quantified in a simulation study. Potential savings in excess of 5% are predicted by more efficient feed material utilisation and by preventing unscheduled delays.