A kinetic study and a~ialysis has been n~acle of the effects of ionic strength. acid activity, temperature, and salt type on the dissolution of magnesium in acidic salt sol~~tions. This is an exanlple of the simplest type of corrosion involving hydrogen evolution. The results are interpreted in terms of the effects of the various factors on the structure of a surface film which must be nlagnesil~rn oxide and/or hydroxide even in acidic solutions. The importance of internal dissolutions in the film a t high concentrations of attacking reagent, for this a~~d other cases, is shown. Owing to co~nplex formation, under certain conclitions a n odd case of "chemical control" of the dissolution rate in this sirnplest case bccomcs evide~it. Corrosion pote~ltial measurements aid in the interpretation. INTRODUCTIONAccording to King and co-workers (12), who support a modification of the diffusion-layer theory originally due to Noyes and Whitney (IS), and Nernst (17), the experinlental support for probable diffusion control of the rate of dissolution of metals in acidic aqueous solutions rests on the following facts. For convenience these are reproduced in part from the recent paper by Roald and Beck (19).1. Different solids dissolve a t nearly the same rate in the same reagent under the same conditions.2. Increased stirring increases the rate. That the rate of dissolution in many cases is a linear function of linear velocity, except a t very low linear velocity, has been demonstrated for some cases by the data of Icing et al. 3. The rate of dissolution has been found for many cases t o be nearly inversely proportional to the viscosity of the solution for a number of solutions.4. The rates of dissolution in acid solutions seen1 to follow the diffusion coefficients of the acid through the solution, perhaps even more closely than they follow the acid strength.5. Temperature coefficients have been found by many i~lvestigators to be of the order usually associated with a co~ltrolling step which is some physical transport process. Temperature coefficients are usually of the order of 3000 t o 5000 cal./mole. I t should be noted that these criteria-do not eliminate all but diffusion control. These criteria could apply equally well to ally type of Inass transfer step which is part of the dissolution process. T h a t the rates follow roughly the diffusion coefficients of the acid through the solution indicates t h a t the control of the process is related in some manner to the physical structure of the solutions. However, the rate of dissolution might well be controlled by any physical step whose rate, in turn, is depencle~lt upon the changing structul-e of the surface film. Each of the criteria listed above refers indirectly to the effects produced by changes in the properties of this surface layer. For instance, King and Cathcart (11) measured ( a ) the diffusion coefficient of the acid, and (b) the rates of dissolution of niagnesium in a series of solutions. 'l'he diffusion coefficients of the acids in the bulk solution wer...

AIeasurements have been made of the el'fect of pressure on the rate of dissolution of M g a t pH 2.0 in two concentrations of aclueous MgCla. Over the pressure range 0.026-2.5 atmospheres, the rates in 0.3 and 3.0 lnolal solutions have, respectively, the pressure dependencesThe induction period of the reaction \vas studied in detail ivith a sensitive gas-measuring technique. Mldrogen enters the metal in the early stages, t o about 1 cc per cc Mg a t 27" C. Its rate of release after the specimen is dried is about libe times higher than the rate a t which it enters the magnesium.S -R a y ditkraction patterns of insoluble reaction products formed 011 the metal surface after prolonged reaction were examined. Magnesium and lnagnesium hydroxide are formed in both solutions, while a substantial amount of still unidentilied extra solid material (not one of the hydrated hydroxy chlorides) is formed in the 3 molal solution.

Experimental determinations of conclitions necessary for the electrolytic polishing of amphoteric metals in alkali hydroxide solutions are described. The effects of temperature, electrolyte concentration, current density, and time of current flow are presented, as are differences among lithium, sodium, and potassium hydroxide solutions. Interpretation of the results i!l terms of a theory involving anodic polarizations has indicated the importance of the formation and maintenance of an anodic film whose electrical and mechanical properties determine critically the nature of the surface resulting fro111 anodic treatment. Further applications of such studies toward the elucidation of mechanisms of anodic processes are indicated.