Visual guide of zinc for estimation of corrosion levels

It is often very useful to be able to estimate the level of corrosion at a location. In order to get an accurate corrosion level, metallic coupons need to be exposed for significant periods. In the case of outdoor fastener testing in AS3566 a level of zinc corrosion measured by coupons is required to determine the completion of the exposure. In order to get the end point of the test, a number of withdrawals and processing of coupons is required over a period of time to determine the correct corrosion level. Having a quick and simple estimation of the corrosion level would reduce the work needed to determine the end point of the test. Corrosion on zinc is reasonably uniform and the possibility of being able to gauge the corrosion level of a location by a visual appraisal of zinc coupons would seem to be possible. CSIRO has exposed zinc coupons in the outdoors for more than 20 years and has collected photographs and corrosion levels over a wide range of corrosion sites. This paper investigates the possibility of a visual guide to corrosion levels using zinc coupons of known composition and corrosion loss and how this may be applied to other materials.