G.T. Burstein

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.

The Effects of Bicarbonate on the Corrosion and Passivation of Iron

Abstract The corrosion of iron in bicarbonate solutions has been investigated potentiostatically and by Auger electron spectroscopy. The dissolution of iron in the active and prepassive regions is greatly accelerated by the presence of bicarbonate, owing to the formation of the stable soluble complex anion Fe(CO3)22−. A mechanism is proposed for the dissolution process. Pitting of the metal surface is observed under conditions where dissolution as the complex occurs. Raising the potential into the passive region where Fe(III) forms in the film prevents complexing and so anodically protects the metal from pitting. Possible implications of complex formation in the stress corrosion cracking of mild steel in bicarbonate electrolytes are discussed.

Reactions of pipeline steels in carbon dioxide solutions

This paper describes the effects of dissolved carbon dioxide on the electrochemical reactions of two pipeline steels: a low alloy steel and a 13% Cr stainless steel. The dissolved gas reacts with the iron component of these steels in such a way as to contribute to their depassivation. The mechanism proposed is that of complexing with Fe (II) within the hydroxide film to give both ferrous carbonate and a dissolved complex. The effect is negligible for the 13% Cr stainless steel, but contributes to the corrosion of the low alloy steel. The cathodic regime representing evolution of hydrogen is also affected by the presence of dissolved carbon dioxide. At low cathodic overpotentials this appears to arise from a pH buffering action. At high cathodic overpotentials there is some evidence that the carbon component of the dissolved CO2 is reduced, probably to carbon monoxide, and this reduces the rate of hydrogen evolution by cathode poisoning. The roles of pH buffer and of electrolytic anion are discussed.

Aspects of the anodic oxidation of methanol

Abstract This paper describes some aspects of recent investigations into the anodic oxidation of methanol. Methanol has long been proposed as an anode fuel for a fuel cell, chiefly because of its ease of carriage, distribution and manipulation. However, methanol is very much more difficult to oxidise anodically than hydrogen, the more conventional anode fuel, and this has hampered development of commercial direct methanol fuel cells. Platinum-ruthenium catalysts are the most active discovered to date. Some advances in electrocatalysis of the methanol reaction by non-noble materials are discussed.

Origins of pitting corrosion

Abstract Corrosion of metals and alloys by pitting constitutes one of the very major failure mechanisms. Pits cause failure through perforation and engender stress corrosion cracks. Pitting is a failure mode common to very many metals. It is generally associated with particular anions in solution, notably the chloride ion. The origin of pitting is small. Pits are nucleated at the microscopic scale and below. Detection of the earliest stages of pitting requires techniques that measure tiny events. This paper describes techniques designed to do this and discusses the measurements that result. Some metals show preferential sites of pit nucleation with metallurgical microstructural and microcompositional features defining the susceptibility. However, this is not the phenomenological origin of pitting per se, since site specificity is characteristic only of some metals. A discussion is presented of mechanisms of nucleation; it is shown that the events are microscopically violent. The ability of a nucleated event to survive a series of stages that it must go through in order to achieve stability is discussed. Nucleated pits that do not propagate must repassivate. However, there are several states of propagation, each with a finite survival probability. Several variables contribute to this survival probability. Examples are shown of several metals and some common features of their behaviour are discussed. It is shown that for some systems, the pit sites can be deactivated.

Electrocatalysts for fuel cells

Abstract A brief description of the six main types of fuel cell which are currently under research and development is given. The discussion focuses on recent developments in the polymer electrolyte-based proton exchange membrane fuel cell with description of the limitations imposed by current electrocatalysts and the benefits offered by the development of improved materials.

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.

The role of alloyed molybdenum in the inhibition of pitting corrosion in stainless steels

The study of both nucleations and metastable pitting events in stainless steels was carried out in acidified chloride solution. The results presented below show that the presence of molybdenum as an alloying element in stainless steels reduces the incidence of both nucleations and metastable pits. In addition to this, the extent of growth these events achieve is reduced and the attainment of stable pit growth made more difficult by the presence of molybdenum.

Effect of impact angle on the slurry erosion–corrosion of 304L stainless steel

Abstract This communication describes an investigation of the effect of impact angle on slurry erosion–corrosion of stainless steel using a new slurry erosion rig. With the new apparatus, it is possible to measure the individual erosion events under impact as both electrochemical current/time transients, and mechanical transients through acoustic emission (AE) simultaneously. Each sharp rise observed in the electrochemical current transient under particle impact is accompanied by an AE event. These sharp current rises are attributed to the rupture or removal of the oxide film on the surface by the abrasive particles. The correlation between the current rise and the maximum of the AE event, although scattered, shows an increase with decreasing impact angle. The current transients due to individual erosion events show that on the average, the current rise is greater and the rise time is longer at oblique angles compared with those at normal incidence. The degree of denudation of the metal surface by individual particle impingements, the process which strips the surface of its passivity, increases at more oblique angles. Weight loss measurements demonstrate that the synergistic effect between erosion and corrosion is enhanced by a more oblique angle of impact. The origin of the synergism is discussed.