Effects of microstructure on the corrosion behavior of CoCr porous coatings on orthopedic implants

Abstract

Cobalt-chromium alloy porous coatings have become increasingly popular as a means of achieving a stable, longer lasting fixation on orthopedic implants. However, sintering heat treatments cause changes in the microstructure that result in changes in the corrosion behavior of the porous coatings. Experiments were conducted to examine the effects of microstructure on the corrosion of CoCr porous coatings. Four distinct microstructures were characterized. Light microscopy revealed microstructures A and B had relatively fewer carbides and a large-grain structure, microstructures C and D displayed a finer grain size with significant carbide formation, predominantly within grains and fusion zones. Accelerated anodic corrosion experiments were conducted to study the localized attack of each microstructure. Experiments were conducted using metallographically polished porous-coated disks for durations of 1, 4, 16, 24, and 72 h. The disks were examined with SEM and EDS to observe preferential attack and element depletion. Results showed a progressive dissolution of the matrix, with preferential attack of the grain boundaries and regions adjacent to the carbides due to sensitization. A precipitation layer was found to be Cr-rich and possibly composed of chromium hydroxide or a chromium orthophosphate. In addition, the solution became yellow in color with longer exposure times, possibly due to the presence of chromate ions, as indicated by an increased chromium level detected by AAS.

In-Vivo Corrosion and Fretting of Modular TI-6AL-4V/CO-CR-MO Hip Prostheses: The Influence of Microstructure and Design Parameters

“…These carbides have excellent corrosion resistance at the expense of the matrix which has been partially leached of its primary corrosion resistant additive, chromium [63]. This process, known as sensitization, causes the chromium deprived areas to become favorable sites for localized corrosion attack [32]. Although the primary purpose of Co-Cr homogenizing solution treatments is to improve the poor mechanical properties seen in the as-cast state, the resulting microstructure homogenization has also been found by various groups to increase localized corrosion resistance.Ti-6Al-4V is not prone to sensitization-like effects as seen in Co-Cr.…”

“…In the case of solution treated cast Co-Cr, care must be taken to avoid undoing the advantages offered by solution treatment i.e. reprecipitating the various second phases and inducing further grain growth [32]. The work strengthened nature of wrought Co-Cr means that the exposure to the high temperatures required for the coating application will release all the stored deformation energy, negating the advantages provided by wrought processing [20,33].…”

Fabrication and Mechanical Properties of Porous Co–Cr–Mo Alloy Compacts without Ni Addition

“…3) Therefore, the porous Co-Cr-Mo coating on the implants as well as the substrate Co-Cr-Mo alloy, should not contain Ni since the release of metal ions at the large interface between body fluid and metal 4) presents great concern.…”

Electrolytic Al2O3 coating on Co-Cr-Mo implant alloys of hip prosthesis

“…However, the ion-release mechanism has shown that the amounts of each element released were not proportional to the composition in all cases. 8,9 Furthermore, it has been indicated that corrosion rates of CoCr implant alloys in vivo are much faster than in vitro. 10 To solve the above problems, such as wear debris and ion release, some improvements have been accomplished.…”