The initial stages of corrosion of iron by unstirred saturated aqueous H2S solutions at 21~ and atmospheric pressure have been examined as a function of time, pH (from 2 to 7, adjusted by addition of H2SO4 or NaOH), and applied current. Detailed examination of the morphology and phase identity of the corrosion products has led to a qualitative mechanistic understanding of the corrosion reactions. Mackinawite (tetragonal FeSl-x) is formed by both solidstate and precipitation processes. Cubic ferrous sulfide and troilite occur as precipitates between pH = 3 and pH = 5, subsequent to metal dissolution upon cracking of a mackinawite base layer formed by a solid-state mechanism. The corrosion rate, and the relative amounts of these phases produced, are controlled by pH, applied current, and the degree of convection. The corrosion rate increases with decreasing pH; the quantity of precipitated material peaks near pH --4, below which dissolution becomes the dominant process as the solubilities of the sulfide solids increase. Significant passivation was observed only at pit = 7, when the initial mackinawite base layer remained virtually intact. The solid-state conversion of cubic ferrous sulfide to mackinawite at 21~ was monitored by x-ray diffractometry. The resulting kinetics are consistent with the Avrami equation for a nucleation and growth process with a time exponent of 3.The initial corrosion of iron or carbon steel by hydrogen sulfide saturated water at low temperatures is complex (1-5). There is considerable confusion regarding product identity in the earlier literature (5, 6) but it is now established that the initial corrosion involves the formation of up to three iron monosulfide phases (1-4): mackinawite (tetragonal FeSI-x), cubic ferrous sulfide, and troilite (stoichiometric hexagonal FeS). Solubility measurements (7, 8) and interconversion studies (1, 2, 9) indicate the order of stability troilite > mackinawite > cubic ferrous sulfide. Troilite is a stable phase in the Fe-S system (10), but it is metastable with respect to pyrite in the solutions used in the present study (11). Mackinawite appears to be metastable under all conditions, but it occurs widely both naturally (9, 12) and as a precipitate under a wide range of laboratory conditions (8, 13) as well as in corrosion products. Little is known about cubic ferrous sulfide, other than its occurrence as a corro= sion product, and its ease of conversion to mackinawite (1, 2).Although there is a substantial body of literature describing the occurrence of these phases, no systematic examination of the chemical conditions of their formation on corroding surfaces has hitherto been reported. The purpose of the present study was to make such an examination, and to elucidate the mechanisms whereby the three solid corrosion products are formed. We have therefore examined the sequence of corrosion of iron in unstirred saturated aqueous H2S at 21~ as a function of pH, reaction time, and applied current.This work formed part of a study of corrosion and deposition p...

The anodic oxidation of copper in LiOH solution has been investigated by galvanostatic, potentiostatic, and voltammetric sweep techniques. The structure and composition of the films were determined by x-ray and electron diffraction, and by scanning electron microscopy. Cu(OH)2 forms in two layers: a base layer grown by a solid-state mechanism and an upper layer of individual crystals nucleated and grown from solution. The size and number of upper layer crystals are dependent on electrode potential. More anodic potentials produce a large number of randomly deposited crystals, whereas less anodic potentials result in fewer, more highly developed crystals. Increased stirring results in a greater loss of material into solution, and in the extreme, nucleation and growth are completely prevented. For sufficiently low crystallization rates, produced galvanostatically, the thermodynamically stable phase, CuO, is formed. At higher rates the formation of Cu(OH)2 dominates. A nucleation and growth mechanism is given and discussed with reference to other metal systems. ExperimentalMeasurements were made on copper electrodes (99.99% purity) of 5.0 cm 2 geometric surface area. Rotating disk experiments were made on 2.5 cm 2 copper disks embedded in Teflon. Samples were polished

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The electrochemical behavior of iron in 1M LiOH solution at 22 ~ and 200~ has been investigated using cyclic voltammetry, galvanostatic charging, and steady-state polarization techniques. At 22~ the data are consistent with the initial formation of Fe804 and Fe (OH)2 on the surface followed by the oxidation of iron to -y-Fe~O3 or Fe(OH)s. The anodic behavior of iron at 200~ can be explained by the following processes 1 ]Present address:

Pubiication costs assisted by Atomic Energy of Canada Ltd.The hydrothermal hydrolysis of A13+ in aqueous KC1 solutions and the precipitation of boehmite (7-A100H) have been studied by in situ acidity measurements at temperatures to 200". The effect of varying the initial solution pH and the total aluminum concentration was investigated. The hydrothermal hydrolysis behavior was found to be consistent with the equilibria 2A13+ + 2H20 = A12(OH)24+ + 2H+ and 14A13+ + 34H20 = A114(OH)34~+ + 34H+ followed by the irreversible formation of boehmite from the polymeric species AIl4( OH)348+. IntroductionIt has long been recognized that oxides and hydroxides can be precipitated hydrothermally from solutions containing dissolved metal ions. This phenomenon is of considerable scientific and practical interest since it is believed to occur in processes such as the formation of terrestrial ore deposits28 and the growth of oxide layers on corroding metal surfaces.2b Helgesonza has recently reviewed previous work on the hydrothermal deposition of solid products from ionic solutions. These studies show that the deposition process is largely determined by the effect of temperature on the position of equilibria between metal ions and complexing species in solution. The equilibria of interest in the present study involve the hydrolysis of metal ions to form progressively higher hydrolyzed species and ultimately the solid oxide or hydroxide. While it is known that these equilibria shift in the general direction of the solid product with increasing temperature, few details of this process have been determined. For instance, almost no data are available on the identity of the hydrolyzed species in the system prior to precipitation or as to how temperature affects the pH of the solution and the average number of hydroxide ions bound per metal atom in the system. These data are required before a quantitative description of hydrothermal precipitation is possible.In this study we have used recently developed potentiometric techniques334 to measure the acidity of aluminum chloride-potassium chloride solutions at temperatures to 200". The hydrolysis and precipitation phenomena have been studied as a function of initial solution pH, total aluminum concentration, and time of storage of the solution at 25". Detailed X-ray diffraction studies of the precipitated phase were made to supplement the aciditytemperature measurements outlined above.