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For many years, use of 30:70 solder (30 percent tin, 70 percent lead) was common in diesel truck radiators. The 30:70 alloy was relatively brittle, however, and became prone to stress cracking, a result of vibrational stresses built up in the radiators. This led to failure of soldered joints, causing leaks and eventual failure of the engine. At engine manufacturers' requests, solder producers worked to develop a better solder alloy. Eventually a soft solder alloy was produced which could better withstand engine vibrations. This soft solder, sometimes called silver solder, has the following approximate composition: 97 percent lead, 1.5 to 2.5 percent tin, and 0.5 percent silver.
Dearborn Chemical (U.S.) has had a long-term treatment development program for closed cooling systems, resulting in an inhibitor which has given excellent performance in the field. This inhibitor also performs well when screened by ASTM Corrosion Test for Engine Coolants in Glassware (D 1384-70). Shortly after engine manufacturers began to use the new soft solder alloy, reports of severe solder corrosion came in from the field. Laboratory tests were then done using soft solder. Initial data developed by ASTM D 1384 showed that tests using soft solder specimens gave only slightly higher solder corrosion rates than tests using 30:70 solder in inhibited standard corrosive water. Further testing, however, did not reproduce data similar to the first set of data obtained using soft solder.
Because reproducibility of screening test results using soft solder was poor, a study was undertaken to examine variability of soft solder corrosion results.
Tests performed were based on ASTM D 1384. Data developed lead to two basic conclusions. First, corrosion rates for soft solder tend to be higher than those for 30:70 solder and corrosion rates of soft solder deviate much more from a mean value (for a given inhibitor dosage) than for 30:70 solder. This holds for water coolants and water-ethylene glycol coolants, both inhibited or uninhibited. Second, the water-ethylene glycol coolant is much more aggressive to both solder alloys than is standard corrosive water. This is the case for inhibited and uninhibited solutions.
solder, engine coolants, statistical analysis
Research chemist, Dearborn Chemical, USA, Lake Zurich, Ill.