D. Landolt

Passive films on stainless steels—chemistry, structure and growth

Abstract The outstanding corrosion resistance of stainless steels results from the presence of a thin oxide—‘passive’—film on the metal surface, typically 1–3 nm thick. The characterisation of the composition and structure of such thin films and the study of their interaction with corrosive environments requires a combination of sophisticated experimental techniques. This paper reviews progress in the characterisation and understanding of passive films on stainless steels achieved over the past two decades. During this period, ex situ surface analysis methods have made substantial progress and new in situ methods for the study of passive films with atomic resolution have been introduced, giving real time information on film chemistry and growth. It has been found that whereas passive film growth occurs in seconds or minutes, long range film ordering is a considerably slower process that takes several hours. In situ investigations indicate that at short times, charge transfer at the metal/film or the film/solution interface limits the rate of film growth on stainless steels. In situ estimates of film composition confirm previous data obtained with ex situ techniques.

Fundamental aspects and applications of electrochemical microfabrication

Abstract The theory and applications of electrochemical microfabrication technology are reviewed focusing on electrodeposition and dissolution processes. Electrochemical microfabrication offers some unique advantages over competing vapor phase technologies and therefore finds increasing use in the electronics and microsystems industries. The present paper discusses the underlying principles of electrochemical microfabrication processes. The important role of mass transport and current distribution is stressed and it is shown how numerical modeling contributes to the present understanding of critical process parameters. The application of electrochemical microfabrication technology in the electronics industry is illustrated with selected examples.

Fundamental aspects of electropolishing

Abstract The literature on the mechanism of electropolishing is reviewed. A brief historical outline is given and anodic levelling or macrosmoothing is distinguished from anodic brightening or microsmoothing. Macrosmoothing results from differences of the gradient of electrical potential and/or concentration on a rough surface. Its rate can be quantitatively predicted in many cases. Microsmoothing results from the suppression of crystallographic factors on the dissolution process. Optimum conditions are achieved when dissolution is under mass transport control. The role of anodic films and of pitting is discussed.

Electrochemical micromachining, polishing and surface structuring of metals: fundamental aspects and new developments

Electrochemical micromachining (EMM) has emerged as a versatile process for machining and surface structuring of metallic materials for biomedical and microsystems applications. From a fundamental point of view EMM presents many similarities with electrochemical machining (ECM) and electropolishing (EP) provided one takes into account the scale dependence of phenomena. In the present paper the role of mass transport, current distribution and passive films for shape control and surface smoothing is discussed and illustrated with examples. The usefulness of numerical simulation using simplified models is stressed. New developments in EMM of titanium are presented, including oxide film laser lithography permitting EMM on non-planar surfaces without photoresist and the fabrication of two-level and multi-level structures. Scale resolved electrochemical surface structuring of titanium leads to well-defined topographies on the micrometer and nanometer scales, which are of interest for biomedical applications.

Induced Codeposition I. An Experimental Investigation of Ni‐Mo Alloys

The electrodeposition of nickel-molybdenum alloys was studied on rotating cylinder electrodes. The current density, electrode rotation rate, electrolyte temperature, and species concentrations were shown to influence alloy composition. The mass-transport limiting species were identified for different operating conditions and electrolyte compositions in order to study the rate-limiting steps of induced codeposition. If the concentration of nickel in the electrolyte was much larger than that of molybdate the molybdenum content in the alloy increased with rotation rate. On the other hand, if the concentration of molybdate in the electrolyte was larger than that of nickel the alloy composition was found to be independent of rotation rate. These results were applied to the deposition of compositionally modulated Ni-Mo alloys exhibiting larger periodic variations in Mo concentration than hitherto reported.

Induced Codeposition III. Molybdenum Alloys with Nickel, Cobalt, and Iron

A comparison was made between the codeposition behavior of NiMo, CoMo, and FeMo alloys on rotating cylinder electrodes when the molybdate concentration in the electrolyte was much lower than that of the iron-group species. More molybdate was codeposited with Co than with either Fe or Ni from an ammonia-citrate electrolyte at pH 7.4. During the NiMo codeposition, the molybdate species is mass transport controlled. Substitution of the nickel by cobalt in the plating bath does not influence the molybdenum deposition rate. However, a higher concentration of molybdate is found in the deposits because the rate of cobalt deposition is lower than that of nickel. On the other hand, substitution of the nickel by iron results in a dramatic lowering of the molybdenum deposition rate. These observations were described by a mathematical model which assumes that iron-group species can adsorb on the electrode surface, competing with the molybdenum intermediate for free surface sites. Thus the diminished partial current density of the molybdenum in iron containing electrolytes can be explained by a blocking mechanism due to the adsorbed iron intermediate.

Wear‐Accelerated Corrosion of Passive Metals in Tribocorrosion Systems

A model is presented which describes the effect of mechanical and materials parameters on the wear-assisted corrosion rate of passive metals under sliding wear conditions. The model is based on a consideration of contact between the sliding surfaces at multiple asperities and it takes into account the passivation behavior of the metal. Wear experiments were carried out in a reciprocating pin-on-plate tribometer permitting the control of mechanical and electrochemical conditions. An alumina pin was rubbed on nickel, chromium, stainless steel, and titanium alloy plates, in sulfuric acid or sodium sulfate solution. The relative importance of mechanical and electrochemical metal removal was evaluated while applying an anodic potential. Additional experiments were performed under cathodic polarization. The results show that the proposed model can describe correctly the effect on dissolution rate of different mechanical parameters such as applied normal force, stroke length, frequency, and sliding speed. Qualitative agreement was observed with the predicted effect of the materials parameters hardness and passivation charge, but uncertainties concerning the real value of passivation charge and, in some cases, wear of the alumina pin limit the predictive capability of the model when comparing different materials. The experimental results obtained in this study demonstrate that to understand the mutual interactions between mechanical and electrochemical parameters affecting wear-accelerated corrosion it is necessary to look at the tribocorrosion system as a whole.

Induced Codeposition II. A Mathematical Model Describing the Electrodeposition of Ni‐Mo Alloys

A steady-state mathematical model was developed to predict the behavior of the induced codeposition of Ni-Mo alloys in the kinetic and mass-transport controlled regions on rotating cylinder electrodes. The kinetic regions were characterized by a simple Tafel expression. A Nernst boundary layer representation described the mass transfer of ions through a diffusion layer. The governing features of the induced codeposition mechanism included soluble nickel acting as a catalyst to the molybdenum deposition and the generation of an absorbed intermediate species on the electrode surface. The resulting alloy composition was simulated for two electrolytes over a wide range of current densities and electrode rotation rates. The model predictions agreed with the observed trends in the experimental data.

Anomalous Codeposition of Iron Group Metals: I. Experimental Results

The codeposition behavior of three alloy systems, FeNi, FeCo, and CoNi, was studied in acid sulfate electrolytes using rotating cylinder electrodes to control mass transport conditions. The partial current densities of the codepositing metals and of the side reaction were determined from an analysis of the deposit and were compared with those measured for the pure metals deposited under identical conditions. Results confirmed that Ni is inhibited by the presence of and ions. Fe deposition rate is enhanced by the presence of and ions. The ion of the less noble metal has a stronger influence on the deposition behavior of the partner ion. Co is inhibited by codeposition of ions and enhanced by ions. The present data demonstrate that anomalous codeposition of iron group metals involves both inhibiting and accelerating effects.

Electrochemical machining under pulsed current conditions

Abstract The possibility of using pulsed current in electrochemical machining at low electrolyte flow rate has been investigated. Theoretical aspects of predicting electrolyte heating and limiting rate of mass transport are discussed in terms of simplified models. High rate dissolution of nickel in sodium chloride solutions under pulsed current conditions was investigated in a flow channel cell by studying the influence of different pulse parameters on anode potential, surface microtexture, surface roughness and current efficiency of metal dissolution. Obtained results indicate that anode potential and surface finish are controlled by mass transport in agreement with steady state behavior. Maximum current density applicable under pulsed current conditions is limited by the occurrence of sparking.