Correlation between accelerated tests and real life (Topic 11142)

A field test in the actual environment will always give the most reliable corrosion data. These tests give realistic data, but are very time-consuming. That is why accelerated corrosion tests have to be developed. These tests are particularly useful for ranking alloys and surface treatments, but can also give an indication of service life of certain alloys in certain environments if reliable correlation between accelerated tests and real life can be established.

Testing of galvanic corrosion rates gives additional challenges. The area ratio anode /cathode is very important, but the available cathode area will depend very much upon the conductivity or salt concentration of the electrolyte. In a field test under a car in Sweden, a cup of an AlSi1Mg alloy coupled to a seven times larger steel sheet had an average corrosion rate of 45 µm/year (40 000 km). A neutral salt spray test has been performed, 1 hour spray and 23 hours in air of 80% humidity at 35ƒC, with the same specimen geometry, but different area ratios and salt contents. The correlation between the field test and the salt spray tests is shown below.

As expected it seems that the area ratio will be less important when the salt concentration is low, thus only a small part of the steel can be utilised as cathode because of the low conductivity of the electrolyte.

Other investigations on certain cyclic wetting and drying test have shown acceleration factors of about 50, thus one week of testing is similar to one year in service. However, there are very seldom direct correlations between accelerated tests and performance in service.

Acceleration factors achieved in salt spray tests, 1 hour spray and 23 hours in air of 80% humidity at 35ƒC, compared to under car field tests, based on average corrosion rates at different salt concentrations and area ratios.[54]