By means of thermal analyses, microscopic examinations, and X-ray analyses of nitrided electrolytic iron specimens a study has been made of the iron-nitrogen system. From the data obtained and with the diagrams of Sawyer and Fry as a basis, a modified iron-nitrogen constitution diagram has been tentatively drawn. The upper temperature horizontal noted by Sawyer was also observed, but this has been ascribed to a peritectoid instead of a eutectoid transformation.

The Preece test is in common use for inspection purposes to determine the uniformity of galvanized coatings, even though the results are sometimes erratic.An experimental study made with wires with different types of zinc coating and with wires consisting of commercially pure zinc and iron indicates that the pH value of the copper sulphate solution, the rate of solution of the zinc coating, and the cleanness of the test specimens are important factors in the Preece test. The use of cupric hydroxide instead of cupric oxide for neutralizing the copper sulphate solution is recommended, since a stable pH is much more quickly reached by its use.Electroplated and sprayed zinc coatings dissolve more rapidly in the copper sulphate solution than do hot-dipped or galvannealed coatings. Pure zinc coatings showed, during their solution, a "potential minimum" (with respect to a copper wire) which was practically absent with coatings that contain iron, such as on galvannealed wires and the inner layer of hot-dipped galvanized wires. This minimum may be caused by the delayed coagulation of basic zinc compounds which coagulation may be accelerated by the presence of iron. The causes and remedies of premature and delayed endpoints are explained. A method for measuring the thickness-distribution of the zinc coating by the use of a suitably acidified copper sulphate solution is suggested.

A quite complete, though brief, review of the literature on equilibrium dia-

For the purpose of obtaining comparative data on the effects produced by different testing methods upon the determination of corrosion resistance of metals, tests were carried out on a copper-nickel series, consisting of copper, nickel, and three copper-nickel alloys, all of commercial grade.The testing methods tried out were simple immersion in nonaerated and aerated solutions, repeated immersion -both continuous and intermittent -spray, and an accelerated electrolytic test.The corrosive solutions, all of normal strength, employed were hydrochloric and acetic acids, sodium and ammonium hydroxides, sodium chloride and potassium dichromat e (used as an oxidizing agent). A few supplementary simple immersion tests were made in which nitrogen gas replaced air for agitating the solution. The corrosion rate and the order of relative corrodibility of the five materials were found to vary very considerably for different test methods.The results clearly indicated that the essential features of the service to be met should be incorporated in any corrosion test designed to give information concerning the suitability of a metal for some particular type of service. CONTENTS Page

The merits of two certain types of accelerated laboratory testing methods for evaluating the indicated life of the coating on hot-dip zinc-coated sheet steel were compared. The two methods studied were the simulated atmospheric corrosion, using a moist gaseous mixture of sulphur dioxide, carbon dioxide, and air, and the spray, using normal solutions of sodium chloride and ammonium chloride separately. A consistent relationship between the " life" and weight of the coating was shown by the resu lts. The time requ ired for the breaking down of the coating was considerably less for the first method than for the second. The coating was corroded, in the first mcthod, in a progressive manner over the entire surface and similar to that reported for galvanized materials corroded in the atmosphere under service conditions, and in the sccond method, in a local and capricious manner. No attempt was made to interpret the experimental results in terms of service lifc in various types of atmospheres prevailing in different climates. Any satisfactory attempt at such an evaluation will have to await the results of long-time field tests on zinc-coated products carried out under several typical atmospherical conditions obtaining at different locations. The presence or absence of about two-tenths of 1 per cent copper in the steel base produced no apparent effect on the results. Tests were also made on specimens which had been annealed for the purpose of converting the zinc coating into an iron-zinc alloy.

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