Effects of environmental factors on atmospheric corrosion of aluminium and its alloys under constant dew point conditions

Abstract

Environmental factors, such as chloride deposition rate, dew point and the addition of sulphate ions, were investigated under constant dew point conditions. Corrosion mass losses of AA1100 increased with increasing chloride deposition rates, dew points and with the absence of sulphate ions. Sulphate ions were shown to have an inhibition effect on pitting corrosion of aluminium alloys. High-purity aluminium was not sensitive to an increase of chloride deposition or dew point. As dew point increased from 5 to 28 °C, the corrosion mass loss of AA6061 decreased. The pitting susceptibility of AA6061 was rather high under low dew point conditions.

Introduction

Atmospheric corrosion has been reported to account for more failures in terms of cost and tonnage than any other types of material degradation process [1]. Atmospheric corrosion affects all structures exposed to potentially corrosive environments. Aluminium and its alloys are widely used in the transport and electricity industries and also in architecture due to their high corrosion resistance, low density, good electrical conductivity and excellent decorative properties. The rate of atmospheric corrosion of aluminium is low, with the average rate being less than 1 μm year⁻¹ [2], [3], [4], [5].

Atmospheric corrosion typically occurs beneath very thin electrolyte layers which undergo cycles of evaporation and condensation. It is, however, a discontinuous process and the frequency, duration and kinetics of active periods of atmospheric corrosion are controlled by the stochastic changes in the environments. A great deal of effort has been made to derive mathematic formulas expressing atmospheric corrosivity as a function of “environmental factors”, such as relative humidity, deposition of aerosol salt particles, time of wetness, pH of precipitation and pollutants. The effect of the chloride and SO₂ deposition rate have been extensively investigated in the field exposure tests [2], [3], [4], [5], [6], [7], [8], salt spray tests [9], [10], [11], and constant humidity tests [12], [13], [14]. The higher the chloride deposition rates (i.e. in the coastal environments) and the higher the SO₂ deposition rate (i.e. in the industrial environments) are, the higher the corrosion rates commonly occurred [4], [5], [6], [8]. A power law in the form of ΔW = A × tⁿ was proposed to fit the data obtained from the atmospheric corrosion of metals [6], [7], [15], [16], [17]. The value of A and n is dependent on the environmental factors. For example, for zinc, the value of A may be approximated as a linear function of the SO₂ and Cl⁻ deposition per unit surface area [15]. In addition, relative humidity [18], [14] and time of wetness [19], [20] play an important role in the atmospheric corrosion of aluminium and its alloys. Studies of hygroscopic salts and aerosols indicate that salts, and thus the surface on which they deposit, may wet over a range of relative humidity depending on the critical relative humidity of the salt [18], [19], [21]. The thickness of the thin surface electrolyte layer beneath which atmospheric corrosion proceeds are influenced simultaneously by relative humidity, deposited aerosol salt, and also the presence of complex corrosion products.

Conventional laboratory simulation tests, such as the salt spray tests, constant humidity tests and cyclic wet–dry tests, tend to have mismatched the relative changes between relative humidity and temperature. In fact, our group [22], [23], [24] reported that the dew point in the ambient atmosphere and near the surface of stainless steel, aluminium and magnesium remained at a constant level during the daily condensation and evaporation of the surface moisture electrolyte films. Relative humidity and surface temperature change along the same dew point level in a short period, which established an explicit basis for controlling relative humidity and temperature in accelerated corrosion tests. On the basis of the previous studies, a cyclic wet–dry laboratory test, named constant dew point corrosion test, was designed to keep the dew point constant while varying relative humidity and temperature. This test reliably reproduces the atmospheric corrosion of stainless steel [22], aluminium alloys [23], magnesium alloys [24], and zinc alloys [25], [26] in terms of corrosion mass loss, and corrosion morphology with a reasonable acceleration ratio.

The present study aimed to figure out the effect of the chloride deposition rate and dew point on the atmospheric corrosion of aluminium alloys. In addition, since the corrosion products of aluminium and its alloys formed in the field exposure test were confirmed to be basic aluminium sulphate and aluminium hydroxide in a previous study [23], the atmospheric corrosion of aluminium and its alloys occurred in artificial seawater with and without sodium sulphate was also investigated by constant dew point corrosion tests in order to clarify the effect of sulphate ions.