In vitro corrosion test methods

When dealing with materials for biomedical applications one has to deal with a more complex environment than for technical applications. Although this includes a quite defined surrounding for a specific tissue, the problem is that many of the parameters found in such tissues are not well described. Additionally, the biological reactions to a degrading material are poorly understood. This can be observed, for example, for degradable polymers, of which only polylactid acid (PLA) and polyglycolic acid (PGA) have reached 

The basic in vitro tests derived from teehnieal applications are the salt spray test (ASTM B-117) and the submersion test, performed in saline solutions (3.5% NaCl, ASTM G31-72 (2004) Standard Practice for Laboratory Immersion Corrosion Testing of Metals). Both tests are conducted at room temperature. By such tests the mass loss ean be determined according to equation (10.5). The mass loss is calculated after removing the corrosion products with chromic acid, which at the same time removes the corrosion products and inhibits further corrosion (Forking, 1964), which is suitable for technical applications but may raise problems in biological environments. 

An alternative test is the hydrogen evolution method (Kray, 1934). This test is based on the fact that during Mg dissolution an equal part of hydrogen is produced (equation 10.7). By collecting the evolving hydrogen gas the corrosion rate can be determined easily and rapidly (Song and Atrens, 2003) with material available in most laboratories (Fig. 10.7). 

7 Schematic drawing of the setup for measuring corrosion rates by the hydrogen evolution method. The Mg sample is immersed below a funnel which is directing the gas bubbles into a burette which is filled with a solution. 

One advantage of this method is the option for time-resolved measurements. By measuring at different time points, variations in the corrosion rate can be determined. As the mass loss method is an endpoint measurement, it is not possible to use it for time-resolved experiments. For the measurement of the evolving hydrogen a more sophisticated instrumentation is necessary. Eudiometers (as recommended by DIN 38414-8) are less prone to experimental errors as well as being highly standardized and calibrated. They also offer the option for automation which increases the sample number of parallel tmly online measurements. 

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