Dr. Howard W. Pickering

Research abstracts

THE EFFECT OF pH ON IRON DISSOLUTION KINETICS DURING CREVICE CORROSION*

Although the IR mechanism of crevice corrosion is well established for metals such as iron and nickel, the mechanism for many metals and alloys remains disputed or unexplored. For many of these materials, crevice corrosion is, by default, attributed to acidification of an occluded aqueous solution. In many systems, however, acidification has been shown to have a synergistic effect with the E(x) distribution along the crevice wall. This research explores these two compounding effects by utilizing a new experimental setup that should allow the initiation of crevice corrosion by acidification in an experiment controlled by the potential drop. This is more similar to the actual propagation sequence of crevice corrosion, and should lead to a better understanding of how crevice corrosion occurs in the real world, both in systems in which the IR mechanism has been established and those systems where the exact mechanism remains unresolved.

A STUDY OF Cl- ACCUMULATION WITHIN CREVICES IN PURE IRON SYSTEMS*

In order to study the effect of Cl--ion on the stability of crevice corrosion, the relationship between the potential distribution (E(x)) and the profile of Cl- concentration along the crevice wall is being investigated in pure iron in buffer solutions. A fine glass probe was inserted into the crevice through the top opening to measure the potential distribution along the crevice wall. A Cl-ion selective electrode (Ag/AgCl) was used to measure the in-situ Cl-concentration within the crevice. The Cl- ions accumulated on the crevice wall in a region that corresponded to the active peak potential window of the anodic polarization curve. A series of anodic polarization curves were measured with different Cl- ion concentrations to determine the location of the active/passive boundary along the crevice wall from the measured E(x) profile. The polarization curves also show that a decrease in the pitting potential occurs when Cl- ion is added to the blank bulk solution (0.5 M CH3COOH + 0.5 M CH3COONa).

CREVICE CORROSION OF ALLOY T-2205 DUPLEX STAINLESS STEEL*

Crevice corrosion is being studied using a common corrosion resistant alloy, T-2205 Duplex Stainless Steel. The anodic polarization behavior of T-2205 in acidic-chloride media shows a large active region and low passivation current, which suggests that it makes a good candidate for studying the IR Mechanism of crevice corrosion. Unlike the traditional mechanism, which attributes crevice corrosion solely to changes in crevice solution composition, the IR Mechanism relates crevice corrosion to primarily the shift of the electrode potential, E(x), with increasing distance x into the crevice, and secondarily to the crevice solution composition through its effect on modifying the system's polarization curve.

THE HYDROGEN ABSORPTION INTO 1020 STEEL IN THE PRESENCE OF HYDROGEN SULFIDE*

The effect of hydrogen sulfide on the hydrogen absorption reaction on 1020 steel in acidic solutions is being investigated using the electrochemical hydrogen permeation technique. The results show that hydrogen sulfide increases the hydrogen absorption rate. The increase in the hydrogen absorption rate is coupled with a depolarization of the hydrogen evolution reaction. The effect was less pronounced than in the case of pure iron.

HYDROGEN ABSORPTION INTO LOW ALLOY STEEL IN THE PRESENCE OF HYDROGEN SULFIDE*

The effect of hydrogen sulfide on the hydrogen absorption reaction on 1020 Steel with different Cr, Mo, and V composition in acidic solutions will be investigated using the electrochemical hydrogen permeation technique. The objective of this research is to evaluate the effect of different alloying elements on the hydrogen absorption and evolution reactions.

THE EFFECT OF ORGANIC INHIBITORS ON THE KINETICS OF THE HYDROGEN ABSORPTION AND EVOLUTION REACTIONS ON IRON IN THE PRESENCE OF HYDROGEN SULFIDE*

The effect of benzotriazole (BTA) on the kinetics of the hydrogen evolution reaction (HER) and the hydrogen absorption reaction is being studied on iron in acidic solutions in the presence of hydrogen sulfide using the electrochemical hydrogen permeation technique. The results so far show that BTA decreases the hydrogen absorption rate at higher concentrations. The decrease in the hydrogen absorption rate was accompanied by an increase in the cathodic hydrogen overpotential. The latter demonstrates the inhibiting effect of BTA on the HER. Other organic inhibitors will be evaluated in this study.

TIN-ZINC ALLOY ELECTROPLATING AND ITS CORROSION BEHAVIOR*

Sn-Zn alloy has been regarded as a good replacement of cadmium electroplating. A 70%Sn-30%Zn alloy was deposited from a neutral non-cyanide bath. Scanning electron microscopy (SEM) and X-ray diffraction showed that it was composed of Sn and Zn phases. When the plated sample was exposed to a 0.1 M NaH2SO4 (pH 3.6) solution at open circuit potential (OCP), the corrosion rate could be measured by use of an electrochemical quartz crystal microbalance. The OCP results showed that the Zn dissolution reaction was the major reaction occurring at OCP. SEM also showed that without sufficient agitation, the deposit was composed of two layers. Having a much higher Zn content, the upper layer dissolved quickly in the corrosive solution. Sufficient agitation is necessary to get a uniform coating. Linear potential scanning experiments showed that the anodic polarization current of this alloy was rather small in a wide range of potential until it reached the anodic dissolution potential of pure Sn in the same solution.

CREVICE CORROSION OF ALUMINUM 6XXX ALLOYS*

Crevice corrosion is one of the most severe problems in industry. It is a type of localized corrosion in which the metal deteriorates within the crevices at much greater rates than at crevice free surfaces. The mechanism by which crevice corrosion occurs in aluminum and its alloys is still unclear. The aims of this study are: 1- finding an Al/electrolyte system that exhibits crevice corrosion of aluminum in an accelerated (few hours) experiment, and 2- determining if the IR voltage drop mechanism is or is not applicable to aluminum and its alloys. The ammonium nitrate + sodium chloride solution/Al 6XXX alloy (0.71% Cu) system at 90oC showed crevice corrosion within 10 minutes. The crevice corrosion morphology resembles that of other systems except that the deterioration inside the crevice appears as large interconnected pits. The potential drop within the crevice is about 1.5 volts. The effect of the crevice opening confirmed that, the wider the gap opening, the larger the Xpass. The potentiodynamic polarization behavior of Al 6XXX alloy (0.71% Cu) in the chloride/nitrate mixture revealed that the alloy has two active-passive transitions.

IN-SITU STM STUDY ON THE DEALLOYING BEHAVIOR OF Cu3Au *

Dealloying as a corrosion process has been investigated for several decades. But only recently it became possible to observe surface processes on an atomic level. Cu-Au has been widely used as a model system to study dealloying. The advantage of the system is that it shows perfect miscibility and is stable in a wide potential range in aqueous solutions so that a variety of aspects of the process may be investigated. In-situ Scanning Tunneling Microscopy (STM) allows one to study detailed mechanistic aspects while the dealloying takes place, and also allows observation of the surface relaxation processes that take place after the material has been exposed to corrosive conditions.

MODIFICATION OF SURFACES BY ELECTROCHEMICAL ALLOYING*

Underpotential deposition (UPD) of metals on a metallic surface results in a monoatomic layer of the deposited metal. Atoms of this layer may start diffusing into the substrate allowing further material to deposit. Depending on the conditions, it is possible to produce defined alloy compositions as surface alloys. In the system Cu - Au e.g. it is possible to achieve a defined Cu3Au layer on the surface. Even if this process is slow because of the diffusion step it may be used to improve surface properties of metallic materials.

CHARACTERIZATION OF THE INITIAL STAGE OF SELECTIVE DISSOLUTION OF COPPER-GOLD ALLOYS BELOW THE CRITICAL POTENTIAL*

Selective dissolution of Cu from an alloy of Cu-18 at % Au is shown to proceed over the entire alloy surface down through hundreds of atom layers at an electrode potential that is well below the critical potential of the alloy. TEM micrographs obtained on the Cu-depleted surface reveal nearly uniform Moiré patterns characteristic of the superposition of the two layers with similar crystal structures, i.e., the Cu-depleted and the parent alloy. While measurements from selected-area diffraction patterns are not sufficiently sensitive, calculations based on the average absorbed Moiré fringe spacings indicate that the product layer is an Au-rich alloy rather than a pure Au layer. No evidence was found for islands of pure Au or Au alloy. The rate of selective dissolution of Cu from the parent alloy is suppressed by the retarding effect of this Au-rich layer. The results are compatible with a transport mechanism involving outward diffusion of Cu atoms through an Au-rich product layer that has quite a different nature than the crystalline parent alloy. Calculations indicate an effective diffusivity of Cu within the Au-rich product layer of the order of 10-3 cm2 s-1 at 25 oC. The TEM micrographs show a few isolated pits, which increase in number with time, covering less than 1 % of the alloy surface. The pits often are close together as if they are in a precursor state for the formation of clusters of pits.

Last up-date: 18 Mar 2001.