Mara Cristina Lopes de Oliveira

Corrosion fatigue of biomedical metallic alloys: mechanisms and mitigation.

Cyclic stresses are often related to the premature mechanical failure of metallic biomaterials. The complex interaction between fatigue and corrosion in the physiological environment has been subject of many investigations. In this context, microstructure, heat treatments, plastic deformation, surface finishing and coatings have decisive influence on the mechanisms of fatigue crack nucleation and growth. Furthermore, wear is frequently present and contributes to the process. However, despite all the effort at elucidating the mechanisms that govern corrosion fatigue of biomedical alloys, failures continue to occur. This work reviews the literature on corrosion-fatigue-related phenomena of Ti alloys, surgical stainless steels, Co-Cr-Mo and Mg alloys. The aim was to discuss the correlation between structural and surface aspects of these materials and the onset of fatigue in the highly saline environment of the human body. By understanding such correlation, mitigation of corrosion fatigue failure may be achieved in a reliable scientific-based manner. Different mitigation methods are also reviewed and discussed throughout the text. It is intended that the information condensed in this article should be a valuable tool in the development of increasingly successful designs against the corrosion fatigue of metallic implants.

Effect of molybdate on phosphating of Nd-Fe-B magnets for corrosion protection

Nd-Fe-B magnets are highly susceptible to corrosion and need protection against environment attack. The use of organic coatings is one of the main methods of corrosion protection of these materials. Data related to the effect of conversion coatings, such as phosphating, on corrosion performance of these magnets is still scarce. Studies about the effect of phosphating on the corrosion resistance of a commercial Nd-Fe-B sintered magnet indicated that it increases the corrosion resistance of these magnets, compared to non-phosphated magnets. In this study, the solution chemistry of a phosphating bath was altered with the addition of molybdate and its effect on the corrosion resistance of magnets investigated. Sintered magnet specimens were phosphated in solutions of 10 g/L NaH2PO4 (pH 3.8), either with or without molybdate [10-3 M MoO42-], to improve their corrosion resistance. The effect of phosphating time was also evaluated, and specimens were phosphated for 4 and 18 hours. To evaluate the corrosion performance of phosphated and unphosphated specimens, a corrosion test based on monitoring hydrogen evolution on the surface of the magnets was used. This technique revealed that the addition of molybdate to the phosphating solution improved the corrosion resistance of the magnets phosphated by immersion for short periods but had no beneficial effect if phosphated by immersion for longer periods.

Investigation on the Corrosion Resistance of PIM 316L Stainless Steel in PEM Fuel Cell Simulated Environment

Bipolar plates play main functions in PEM fuel cells, accounting for the most part of the weight and cost of these devices. Powder metallurgy may be an interesting manufacturing process of these components owing to the production of large scale, complex near-net shape parts. However, corrosion processes are a major concern due to the increase of the passive film thickness on the metal surface, lowering the power output of the fuel cell. In this work, the corrosion resistance of PIM AISI 316L stainless steel specimens was evaluated in 1M H2SO4 + 2 ppm HF solution at room temperature during 30 days of immersion. The electrochemical measurements comprised potentiodynamic polarization and electrochemical impedance spectroscopy. The surface morphology of the specimens was observed before and after the corrosion tests through SEM images. The material presented low corrosion current density suggesting that it is suitable to operate in the PEM fuel cell environment.

EIS investigation of the corrosion resistance of uncoated and coated Nd-Fe-B magnets in PBS solution

The aim of this work is to investigate the corrosion behavior of powder metallurgy produced Nd-Fe-B magnets and to evaluate the corrosion protection afforded by two different surface treatments: a phosphate conversion and a non-functional silane (BTSE) layer. The electrochemical tests were performed in a phosphate buffered solution (PBS) at neutral pH, which ionic concentration coincides with that of the human body. The corrosion behavior was monitored by means of electrochemical impedance spectroscopy (EIS) and anodic potentiodynamic polarization curves, and SEM-EDS analyses were used to monitor coating deposition. EIS response has evidenced a porous electrode behavior for the Nd-Fe-B magnets according to the de Levie theory. The results also indicated a good performance of the phosphate layer, whereas the BTSE layer did not improve the corrosion resistance of the magnets. The good anticorrosion performance of the phosphate layer was explained on the basis of the formation of an insoluble phosphate layer both on the electrode surface (identified by interference colors) and on the pore walls. Precipitation of insoluble Nd phosphate on the Nd-rich phase also contributes to the superior corrosion protection afforded by this coating.

The Influence of Al, Cu and P Content on the Magnetic Properties of PrFeCoBNb-Based HDDR Magnets

This paper reports the results of investigations carried out to determine the magnetic properties of some praseodymium-based magnets represented by the formula Pr14Fe63.9-xCo16B6Nb0.1Mx. Bonded magnets of various compositions were prepared from annealed alloys using the hydrogenation, disproportionation, desorption and recombination (HDDR) process. The HDDR powders were isostatically pressed and bonded with cyanoacrylate adhesive to form permanent magnets. The effect of the addition element content on the magnetic properties of these magnets was investigated. The amount of Al, Cu and P addition has a significant effect on the magnetic behavior of these magnets. Under the present processing condition 0.3 at%Al enhanced the intrinsic coercivity to 1.27 and a minimal amount of P improved the remanence to 0.77 T. Copper addition was deleterious to the magnetic properties of the HDDR bonded magnets, independently on the content added.

Corrosion Performance and Cytotoxicity of Sintered Nd-Fe-B Magnets

In this investigation, the corrosion performance of a commercial sintered Nd-Fe-B magnet in a cell culture medium, and the cytotoxic response due to the magnet corrosion products leached into the cell culture medium, have been studied. Localized corrosion was observed and the corrosion products leached into cell culture medium were analyzed by ICP-OES. The magnet showed no toxicity in the neutral red uptake assay used in this investigation contradicting results from literature obtained by another method of cytotoxicity assay.

Effect of Passivation Treatments on the Corrosion Resistance of PIM 316L Stainless Steel in a PEM Fuel Cell Simulated Environment

In this work, the corrosion resistance of passivated PIM 316L stainless steel specimens was evaluated in 1M H2SO4 + 2 ppm HF solution at room temperature during 28 days of immersion. Passivation was carried out in HNO3 and H2SO4 solutions. The electrochemical behavior of the passivated specimens was assessed through electrochemical impedance spectroscopy and anodic polarization curves. Scanning electron microscopy (SEM) was employed to observe the surface of the specimens before and after the passivation treatments. The results pointed to a strong influence of the passivation conditions on the corrosion resistance of the specimens.

An Investigation on the Corrosion Behaviour of Nd-Fe-B Magnets in a Chloride Solution

In this study, the corrosion behaviour of a sintered commercial Nd-Fe-B magnet has been investigated in naturally aerated and deaerated 3.5 (m/v)% sodium chloride solutions. Magnetized specimens were used in this investigation. The corrosion behaviour of the specimens was monitored at increasing test times by means of electrochemical impedance spectroscopy (EIS). Potentiodynamic polarization curves and surface observation by scanning electron microscopy (SEM), were also carried out after 30 days of immersion in the test solutions. Experimental results indicated that the corrosion resistance was affected by aeration of the solution. The corrosive attack penetrates deeply into the specimens and, consequently, the magnetic properties decreased with immersion time. The magnetic properties most influenced by corrosion were the remanence (Br) and the maximum energy product (BHmax), both presenting a reduction corresponding to 6%, after 30 days of immersion.