Alternate wet and dry cycles in the life of a system can create very serious corrosion problems mostly due to the formation of partially dry corrosion products that can further the accumulation of corrosive agents by absorption of moisture and the creation of under deposit corrosion. The severity of the corrosion attack created by alternate wetting and drying cycles can be used to accelerate corrosion and provide a relatively fast estimate of corrosion prevention measures. The ISO 11474 standard, for example, describes an accelerated outdoor corrosion test performed by intermittent spraying of a salt solution onto the exposed pieces of equipment or systems.
In a recent experiment carried out by the Corrosion Doctors, a modified atmospheric corrosion coupon (CLIMAT) was used in an intermittent testing regime to rank the corrosion inhibitive efficiency of corrosion preventive compounds (CPCs).
Atmospheric corrosion test rack for the evaluation of corrosion prevention compounds with a view of historical Fort Henry in the background
Corrosion acceleration by intermittent spraying of a 3% NaCl saline solution
Control corrosion coupon after intermittent spray and exposure for ten days
Protected corrosion coupon after intermittent spray and exposure for ten days
Once cleaned the weight of the aluminum wires of these exposed CLIMATs was measured and compared to the initial weight to provide an estimate of the environmental corrosivity. The corrosion acceleration was quite successful since an average of 15% loss to corrosion was achieved on the non-protected coupons in ten days or 45% per month. In comparison, the most corrosive atmospheric test sites in North America, i.e. Kennedy Space Center corrosion testing facilities, can corrode the aluminum wires at the very fast rate of 30% per month during the most corrosive winter months of the year. Additionally, the coupons coming back from tests at this harshest test site in continental USA look very much like the coupon on the left side of the photos above.
Weight loss and corrosion inhibition efficiency of the nine CPCs tested in this study
Weight loss (%)
The best CPC performer (on the right side of the photos above) only suffered a 0.5% loss over the ten day period, the equivalent of 97% corrosion inhibition efficiency, while the least protective still protected the aluminum with a 54% efficiency.