Protective Coatings: Performance Evaluation and Life Prediction*

Dr. Jianhai Qiu
School of Materials Engineering
Nanyang Technological University
Nanyang Avenue, Singapore 639797

Accelerated Tests for Performance Evaluation

The first and most widely used accelerated test is the conventional salt spray (Fog) test which carries the standard designations of ASTM B117, BS3900-Part12 and ISO 7253. Salt spray was first used in 1914 for corrosion testing and was standardized by ASTM as test method B117 in 1939.  There have been many revisions with the latest designation being B117-97. As a qualifying/acceptance test, it does provide RELATIVE corrosion resistance information for coated/plated metals exposed to constant static condition of 5% NaCl at 35^oC.  There have been many cases where the coated steel lasted thousands of hours in the salt spray tests but failed prematurely in outdoor service (Fig.2) [8]. There are also cases where a coating system performed well in outdoor exposure but failed quickly in the salt spray cabinet.  It has long been recognized that the coating's resistance to the salt spray environment can not be directly translated into the resistance to other environmental conditions. In fact, it states very clearly in the ASTM standard B117-97 that "prediction of performance in natural environments has seldom been correlated with salt spray results when used as stand alone data". The natural environment is a dynamic and ever-changing one. The cyclic wetting and drying when the rain comes and goes, the temperature variation from day to night and the UV radiation from sunlight are all missing links in correlating the salt spray test results with real life performance.