Mechanism of Pit Growth in Homogeneous Aluminum Alloys

Mechanism of Pit Growth

in Homogeneous Aluminum Alloys

G. Knörnschild

Federal University of Rio Grande do Sul

Brazil

1. Introduction

Pitting corrosion is a process, which takes place on passive metals and alloys. A

characteristic of this type of corrosion is that passivity breaks down at isolated points at the

surface and the growth of pits is observed due to locally high rates of metal dissolution. In

electrochemical experiments, the growth of pits leads to a rapid rise of the overall current

density once a characteristic threshold potential, the pitting potential E(pit) is surpassed.

Since the measured current density is composed of the passive current density at the passive

surface area and the current density of fast metal dissolution at the pitted area conventional

electrochemical tests are not useful for studying metal dissolution inside pits. Some authors

tried to overcome this difficulty by working with small wire electrodes. The idea behind was

to achieve an electrode state where the whole surface represents a pit and the measured

current density becomes, therefore, identical to the real current density inside a pit.

However, highly concentrated electrolytes and high potentials must usually be applied to

achieve this electrode state [1-3]. Formation of salt films and mass transport control was

usually observed under these conditions, which more likely represent the conditions of

electropolishing rather than that of a metal suffering pitting corrosion at the corrosion

potential.

Other authors determined the current density during the initial stage of pit growth from the

microscopic measurement of the dimensions of pits formed in short time experiments [4]. By

measuring the time for perforation by pitting of thin metal foils Hunkeler [5] and Cheung

[6] obtained average rates of pit growth, i.e., average current densities normal to the foil

surface. Average rates of localized corrosion at grain boundaries of aged AlCu alloys have

been determined by metallographic measurements of penetration depth and penetration

time [7]. By the same method early stages of pit propagation were studied in 7075-T3 alloy

[8]. Few studies have been made to measure in-situ metal dissolution inside pits.

Edeleanu [9] examined the pit propagation in thin aluminum foils, glued to a glass foil

which could be observed from the back side by a microscope. In this way pit propagation

along the glass foil could be observed in situ. The technique was applied again for an

intensive study of pit growth in pure aluminum by Baumgärtner [10-12], and for the

study of homogeneous aluminum alloys by Knörnschild and Kaesche [13,14]. Later,

Frankel [15] used a similar technique to study pit growth in thin aluminum films

deposited by PVD.

2. Experimental techniques

Pure aluminum as well as homogeneous Al-4wt.%Cu and Al-3wt.%Zn

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