Effect of Nitrate Concentration on Passivation of Aluminum Alloys in Commercial Coolants for Heavy Duty Diesel Engines 900436

Cast aluminum alloys 356 and 319 and wrought alloy 3003 were corrosion tested in a commercial (Fleetguard DCA-4) supplemental coolant additive (SCA) package modified by varying the potassium nitrate level.

Electrochemical techniques were used to determine the stability of the passive film as a function of nitrate concentration. Cyclic potentiodynamic polarization and cyclic galvanostaircase polarization were the principle techniques used and compared. In the presence of the other inhibitors, the passive film stability did not change as the nitrate concentration varied. The corrosion resistance of each alloy was more dependent on the alloy chemistry with 3003 being the most resistant and 319 being the least. The two electrochemical techniques provided results consistent with each other.

With the introduction of advanced materials and processes to the heavy duty diesel engine, the challenge in formulating a coolant is to provide adequate protection for all the materials in the cooling system. Cast and wrought aluminum alloys are increasingly being used or are being considered for use in such components as manifolds, pump housings, and radiators. In the past, silicates have been used in coolants to provide protection to aluminum alloys, however overconcentration of the SCA has lead to silicate dropout in the form of a gel which interferes with coolant flow and heat transfer. The use of a low silicate phosphate/molybdate based additive package is an improvement over the traditional borate/nitrite system, however it still contains substantial amounts of nitrate. Although nitrate is added to improve pitting protection in aluminum, excessive amounts can promote brass cracking.

Gravimetric techniques such as the ASTM D-1384 glassware test, cavitation, and hot surface testing have traditionally been used to develop SCA formulations for heavy duty use. Although electrochemical testing has been used extensively in diverse applications such as nuclear power generation, petrochemical production and industrial coatings development, it has only relatively recently come to be accepted for characterizing material behavior in heavy duty, cooling systems (1, 2, 3, 4 and 5). In addition to the shorter test times for determining an “instantaneous” corrosion rate, valuable information can be obtained as to an alloy's natural passivity and the effectiveness of inhibitors in forming protective films (6). A brief summary of electrochemical testing theory is given in the Appendix and a list of terms and symbols are defined after the Conclusion section.

The purpose of this study is to apply electrochemical corrosion measurement techniques to evaluate the effect of nitrate concentration in Fleetguard's DCA-4 on several typical aluminum alloys, both cast and wrought. It is hoped that the results of this will allow us to specify the minimum amount of nitrate for adequate pitting protection for these alloys. Cyclic potentiodynamic polarization and cyclic galvanostaircase polarization were the primary experimental techniques used to study passivation as a function of nitrate concentration. Potentiodynamic polarization (Tafel plots) were used to measure corrosion current, and hence, rate and polarization resistance. While cyclic potentiodynamic polarization is more commonly used to study repassivation, the kinetic effects on aluminum alloys in particular make the results from this technique dependent on the parameters chosen in making the scan. The pitting potential and the protection potential are dependent, for example on scan rate. To avoid these problems, Hirozawa has proposed the use of the cyclic galvanostaircase technique. This method and its advantages are described in more detail elsewhere (7, 8 and 9). In this study, these two methods will be compared in our specific environment.