Evaluation of the Corrosion Durability of Steel Systems for Automobile Fuel Tanks.

The 2-year corrosion test program included testing in salt solutions to simulate road environments for the exterior of a fuel tank. Special test specimens were designed to represent a manufactured tank. The external tests used were the Neutral Salt Spray test (ASTM B117) with exposures up to 2000 hours and the Cyclic Corrosion test (SAE J2334 ) with exposures up to 120 and 160 cycles to represent vehicle lives of 15 years and 20 years, respectively. Additionally, the resistance to an aggressive ethanol-containing fuel (internal tank corrosion) was assessed by using uniquely designed drawn cups of the various steel systems. Steel cups of each material containing the aggressive test fuel (CE10A) were held at 45°C for 39 weeks to simulate 15-year exposure. Changing the test fuel every 4 weeks simulated the frequent refueling of an automobile fuel tank and served to replenish contaminant ions and minimize oxygen depletion (per SAE J-1747 ).

The criterion selected for failure was perforation, however, visual observation and photographs assessed the progress of corrosion qualitatively. Weight losses and pit depths were used as quantitative assessments. In the external tests, coating integrity was assessed by creepback at a scribe line and chip ratings after gravel impact.

For the simulated external tank corrosion tests the progress of general corrosion (red and white rusting), varied among the materials and with exposure. However, no perforation was observed in any material even after the maximum exposure times. The maximum pit depth observed in only two samples represented about 20% to 40% of the original steel thickness. For the internal tank corrosion tests, no pitting was observed in any of the materials even after 39 weeks exposure.

The detailed results, described herein, demonstrate that several steel systems will meet a 15-year life, and likely will meet a 20-year life. The results are being made available to the automotive community to encourage automakers to conduct their own proving ground tests to validate the SASFT findings.

All authors are current members of the Corrosion Evaluation Team of the Strategic Alliance for Steel Fuel Tanks (SASFT). SASFT is organized by the American Iron & Steel Institute.