Copper deterioration: causes,diagnosis and risk minimisation

Abstract

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Copper is widely used in many applications such as heat exchangers, condensers, heating and air conditioning systems, electricity, electronic circuitry and ornamental parts. This paper reviews the issue of copper corrosion in different environments and applications. Special attention is paid to the influence of relative humidity and low molecular weight carboxylic acids. These acids are present in rain, snow, clouds and particulate matter and contribute about 15-35% of rain acidity. Attention is also paid to copper corrosion originated by formic, acetic, propionic and butyric acid vapours. Corrosion product layers (patina) are analysed, showing the relationship between potential and current density cathodic peaks, and models are proposed that show a proportionality factor of the dimensions of a resistor. These results are of practical importance in copper roofing applications. Before 1985, copper was almost universally considered to be a corrosion resistant conduit of potable water, and few instances of copper tube failure were reported. Where unacceptable performance was identified, the problem was usually attributed to reticulation and plumbing systems. In air conditioning applications, premature failure of copper tubing frequently occurs due to pitting corrosion after a short time in service or even during post-installation leakage tests. This type of corrosion is described in the literature as formicary corrosion, and its morphology is characterised by microscopic caverns connected by tunnels. Specimens are analysed to assess the reasons for such premature corrosion failure. Finally, procedures to mitigate copper corrosion are addressed. Pickling in dilute mineral acids (H₂SO₄ and HCl) is discussed as the most common method for removing oxides formed on the surface of copper based materials during mill processing and manufacturing operations. The use of corrosion inhibitors, such as immersion corrosion inhibitors and volatile corrosion inhibitors (VCI), is a widespread practice to reduce copper corrosion, achieving significant delays in the corrosion process. Reference is made to the search for new environmentally friendly inhibitors for copper and bronze using organic compounds in acid, neutral and alkaline media, and a number of ecological alternatives are reviewed.