Use of Electrochemical Techniques for Corrosion Testing of Antifreezes

Abstract

Two electrochemical techniques were used for corrosion testing of antifreezes. The first method, the galvanic test procedure, evaluates corrosion in galvanic bimetallic couples commonly found in automotive cooling systems. It identifies the anode (major corroding metal) in the couple, gives useful corrosion rates in as little as 24 h, and can generate corrosion rates as a function of time. Thus, more detailed data can be obtained more rapidly than with the standard glassware corrosion test, which gives only a weight loss measurement after two weeks of testing. Test results were compared for a number of antifreezes obtained with this electrochemical method and with ASTM Corrosion Test for Engine Coolants in Glassware (D 1384-70) and ASTM Simulated Service Corrosion Testing of Engine Coolants (D 2570-73). These results show good correlation between the methods but also point out some areas where the electrochemical method can give misleading data. The second method, the resistance polarization method, determines the corrosion rate on a freely corroding metal and is thus used to evaluate corrosion of a metal not galvanically coupled to another metal. An example is shown in which this method was used in combination with the galvanic method to obtain information on a corrosion mechanism; that is, the effect of localized corrosion of one metal on the corrosion losses in a galvanic couple. An antifreeze was selected for which the corrosion losses from the ASTM D 1384 method did not correlate with the corrosion rate measured by the galvanic method. The corrosion rate determined by resistance polarization correlated much better with these weight loss results. This indicates that the mechanism is predominantly controlled by local corrosion processes, which account for the major portion of weight losses seen in the ASTM D 1384 method. The patented corrosion cell used with the electrochemical methods was developed in the BASF Wyandotte laboratory and is designed to be versatile, rugged, and simple. It duplicates the anodic polarization curve for 430 stainless steel in 1.0 N H2SO4 obtained in the ASTM Standard Reference Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements (G 5–72). A discussion on the usefulness of the electrochemical methods for testing antifreezes concludes the paper.