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Volume 2 Extended Abstract 29

Submitted 26th August 1999

Influence of Carboxylic Acids on the Corrosion and SCC Behaviour of Anodic Coated in 3m H2SO4 Al-Alloys

P. Spathis, E. Papastergiadis, G. Stalidis, G. Papanastasiou 
Dept. of Chemistry, University of Thessaloniki, Greece

Keywords: Aluminium alloys, anodizing, substituted carboxylic acids, stress corrosion cracking

Introduction

Aim of the present work is the study of corrosion and stress corrosion cracking behaviour of 1050 Al-Alloy anodised in a 3M H2SO4 anodising bath with the presence in it of various carboxylic acids, saturated or not, with different numbers of carboxylic, methyl or hydroxyl groups. The investigation was carried out by SCC tests and electrochemical measurements. The SCC behaviour of anodised 1050 Al-Alloy was found to vary with anodising conditions, stress level and the differences in the structure and the carbonic chain of the additives. Anodic coatings prepared in 3M H2SO4 without any additives, did not protect the bare alloy. Addition to the anodising solution of any of the carboxylic acids used, had as result a protection of the alloy, with better protective properties in the case of malonic acid and a high stress level. For the interpretation of the results, SEM micrographs and IR spectrum of the surfaces of the specimens were obtained. Properties of the anodic coatings as thickness, packing density, coating ratio, roughness, were also studied. The anodic coatings formed in a electrolytic bath with additives present were found to be less porous, more compact and rough, having better anticorrosive and mechanical properties. An explanation of the mechanism of the effect of carboxylic acids on oxidation and SCC behaviour of Al-Alloys is the absorption of these compounds on the metal surface. Corrosion and stress corrosion cracking behaviour is better in the case of saturated bicarboxylic acids. The presence of methyl groups improves the protective properties of the oxide but increase of the number of these, makes the protective properties of the oxide worse.

 

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