The purpose of this investigation was to determine the corrosion-resistant properties of commercial magnesium-alloy tubing as compared to the corrosion resistance offered by silicone-coated magnesium-alloy, aluminum, monel and stainless steel when used as heat exchanger tubes.

Two single pass, double pipe heat exchangers were constructed using pyrex glass tubes as outer shells. The pyrex tubes were two inches inside diameter, 2.25 inches outside diameter, five feet long and had flared ends. Each end of the pyrex tubes was equipped with a flange containing a 0.876 inch packing gland and a 0.75 inch inlet, or outlet port. The corrosion test tubing could be easily inserted into or removed from the packing glands without damaging the surface of the test specimen.

A series of corrosion tests was made in the heat exchangers using magnesium-alloy, FS-1; silicone-coated magnesium-alloy, FS-1; aluminum, 3S; stainless steel, type 316; and monel as heat exchanger tubes. Three and ten per cent by weight sodium chloride solutions, and sulfur-bearing Texas fuel oil were used as corrosive mediums. The unit was operated at an average inlet temperature of 38 ± 1°C and an average outlet temperature of 50 ± 5°C for the sodium chloride solutions, and an average inlet temperature of 83 ± 3°C and an average outlet temperature of 94 ± 6°C for the sulfur-bearing fuel oil. Seventy-two-hour tests were made maintaining an average rate of flow of corrosive medium through the heat exchangers of 6.7 ± 0.2 gallons per minute for the sulfur-bearing fuel oil.

Upon completion of the tests, the heat exchanger tubes were chemically cleaned of corrosion products. From the known weight losses, area and density of test specimens, and duration of tests, the corrosion rates were calculated. Corrosion rates expressed in inches penetration per year, due to the action of three per cent by weight sodium chloride was as listed in the following descending order: magnesium-alloy, FS-1, pitted, no calculations made; silicone-coated magnesium-alloy, FS-1, 0.1388; aluminum, 3S, 0.0508; monel, 0.0050; and stainless steel, type 316, 0.0013. Corrosion, expressed in inches penetration per year, due to action of ten per cent by weight sodium chloride was as follows: magnesium-alloy, FS-1, and silicone-coated magnesium-alloy, FS-1, pitted, no calculations made; aluminum, 3S, 0.0588; monel, 0.0071; stainless steel, type 316, 0.0036. Corrosion, expressed in inches penetration per year, due to action of sulfur-bearing fuel oil was as follows: silicone-coated magnesium-alloy, FS-1, 1.160; magnesium-alloy, FS-1, 0.897; aluminum, 3S, 0.0361; monel, 0.0095; stainless steel, type 316, 0.0029.

Note: After completion of this thesis, an investigation by John T. Castles concerning the use of silicone coating on steel evaporator tubes indicated that the coating contained minute holes and therefore was not impervious. These holes could have formed at points of weakness and starting points for disintegration of the silicone coating.

It is recommended that the metal surface be treated prior to spraying in an attempt to obtain an impervious silicone coating. The metal surface should be thoroughly dried to insure against moisture remaining in nonconformities which would establish points of weakness under the coating.

Some test should be devised which would indicate whether or not a coating was impervious.