EFFECT OF COLD DEFORMATION ON THE CORROSION BEHAVIOUR OF Mn–CONTAINING ALUMINIUM ALLOY TUBE

Abstract

Aluminium alloys are extensively employed as heat transfer tube materials in heat exchangers.Previous work has focused on the corrosion behaviour of tube materials in brazed heat exchangers.However,very little attention has been devoted to corrosion of tubes in mechanically-expanded heat exchangers despite the observation of some instances of early corrosion failure in bent region of aluminium alloy tube.The effects of cold deformation on the corrosion behaviour of aluminium tube materials have been studied in 0.6 mol/L NaCl and SWAAT solutions by SEM,TEM, potentiodynamic polarizations and immersion test.The results show that high Mn content(0.22%, mass fraction) can experience preferential corrosion and early failure in the bent region,but not for one with a lower Mn content(0.08%).SEM/TEM observations of the microstructure of the alloys show that each alloy has one main type of coarse intermetallic particle.However,TEM observations show that there is a distinct difference in particle morphology between the bent and straight regions of the high Mn alloy tube,the bent region has more nano-scale Mn-rich particles than the straight region, and no such effects are observed on the low Mn alloy.The microelectrochemical polarisation measurements show that the straight region of high Mn has highest pitting potential,but cold deformation can decrease the pitting potential of bent region of high Mn tube,but no such effects on low Mn tube. The immersion test shows that the bent region of high Mn alloy has highest attack;this is associated with precipitation of 20—100 nm Mn-rich particles,which cause increased anodic reactivity as they provide further pit initiation sites and cause solute depletion in the matrix.In addition,the Mn-rich particles are also sites for enhanced cathodic reactivity.The relationship between the microstructure and electrochemical properties of tube materials before and after cold deformation is established.The results indicate Mn can improve the corrosion resistance of aluminium alloy,but the mechanical cold deformation will weaken the effect.