CORROSION RESISTANCE OF ALUMINIUM - Conways

1 First Edition: 2011 AFSAC ompiled and published by theAluminium Federation of South AfricaPocket Guide 3 CORROSION RESISTANCEOF ALUMINIUMAND PROTECTIVE MEASURESWHERE APPROPRIATE CONTENTS1. Introduction to Corrosion2. CORROSION (lamellae attack) Corrosion3. Operating exposed to natural atmospheric ALUMINIUM exposed to ALUMINIUM in engineering applications4. ALUMINIUM and ALUMINIUM of Alloy General Alloy ratings5. Applications and Measures - Normal atmoshperic measures - chemical measures - engineering design2 CCooppyyrriigghhttAll rights reserved.

2 No part of this publication may be reproduced in anyform or by any means, or stored in a data base or retrieval system withoutthe prior written permission of the the best of our knowledge, the information in this book is accurate. AFSA assumes no responsibility whatsoever for errors in, or misinterpretation of,the information in this book or in its RESISTANCE OF ALUMINIUMABSTRACTA luminium and its alloys have excellent RESISTANCE to CORROSION . It doescorrode under some conditions. It is preferable to avoid corrosionthrough appropriate alloy choice and geometric is a function of the operating environment, alloy and the mechanisms of CORROSION enables more appropriatechoices of alloy, geometry and detail in design and obtain the good results that are possible with ALUMINIUM , it is essentialto know the conditions under which CORROSION can occur.

3 ZThe form the CORROSION can takezThe rate or severity of attackzThe preventative measures that can be taken to avoid CORROSION bydesign choices and to prevent premature failure through appropriateprevention detailThis Pocket Guide provides some information on practical corrosionproblems that may arise when using ALUMINIUM . It discusses appropriatepreventative measures to eliminate or inhibit INTRODUCTION TO CORROSIONWhat is CORROSION ? CORROSION is a natural process as nature attempts to return metals totheir original, stable, oxidised state. The degree and severity of corrosionis a function of both the material and its operating has a very high affinity with oxygen.

4 Only Beryllium has ahigher affinity. When a new ALUMINIUM surface is exposed in the presenceof air (or an oxidising agent), it very rapidly acquires a thin, compact,hard, tightly adhering, protective, self healing film of ALUMINIUM oxide(about 0,5 in air). In non-stagnant water, thicker films (of hydratedoxide) are produced. This film is relatively inert chemically. It is onthe inactivity of the surface film that the good CORROSION RESISTANCE ofaluminium depends. When the surface film dissolves, dissolution of themetal ( CORROSION ) occurs; when the film suffers localised damage underconditions when self-healing cannot occur, localised CORROSION may take the form of pitting CORROSION , crevice CORROSION , inter-granularcorrosion or stress CORROSION , depending on the circumstances formation of the oxide is illustrated in Figure 1, and a comparison ofthe oxide characteristics in Figure to oxygen or to an oxidising environment, ALUMINIUM and itsalloys have excellent RESISTANCE tocorrosion.

5 Many of its uses dependon this very property. The surface films are generally stablein the pH range of about 4,5 - are cases beyond this rangewhere the film is inert, , the filmis inert in concentrated nitric acidat pH 0, glacial acetic acid at pH 3and in ammonium hydroxide at pH13. The oxide film is, however,generally dissolved in most strongacids and bases. CORROSION ofaluminium in these is 1: The formation of AluminiumOxide3 Surprisingly, much remains to be learned about the properties andcompositions of the oxide surface film on is the case with all common structural and architectural metals, ALUMINIUM does corrode under some conditions.

6 This especially occurswhen the wrong alloy is chosen, or it is incorrectly used for the obtain the good results that are possible with ALUMINIUM , it is essentialto know the conditions under which CORROSION can occur, the form thecorrosion can take, the rate or severity of the attack and most importantof all, the preventative measures that can be taken. Most corrosive atmospheric environments contain oxides of sulphurthat are by-products from the combustion of oil, petrol or coal. Sulphurdioxide, by far the most commonly occurring oxide of sulphur, isabsorbed from the atmosphere onto most metallic surfaces.

7 This isparticularly so for iron and steel. When conditions are humid, the sulphurdioxide is converted to sulphuric acid, with iron sulphate an intermediateproduct. In the case of ALUMINIUM and other non ferrous metals, thissulphuric acid is absorbed in the CORROSION process. It gives rise to theformation of metallic sulphates. However, in iron and steel the sulphuricacid is regenerated as a result of hydrolysis and the CORROSION the addition of alloying elements to the ALUMINIUM affects itscorrosion RESISTANCE , it is difficult to make general statements of its4 Figure 2:A comparisonof oxidecharacteristicscorrosion performance.

8 To create some degree of order and perspectivein this guide some generalisation must be general, ALUMINIUM alloys have good CORROSION RESISTANCE in thefollowing environments:Atmospheres (rural, industrial, marine):zFresh Waters (natural, treated, reclaimed)zSea Waters (quay side, open water)They also offer good CORROSION RESISTANCE to:zMost soilszMost foods and zMany Chemicals2. CORROSION General CorrosionThe general rate of surface CORROSION in local atmospheres was measuredin various South African inland and coastal centres in 25 year long termstudies by the CSIR. Whilst somewhat more than stainless steels, thegeneral CORROSION of ALUMINIUM was about 20% that of zinc and 1% thatof low carbon Pitting CorrosionPitting is the most common formof CORROSION found in alloys are affected tovarying degrees depending ontheir composition, thermal historyand the environmental pitting is a galvanic reactionbetween elements in the alloy, ingeneral, the purer and simplerthe alloy the higher its resistance5 Figure 3: Typical pitting curves showingpit depth development over time6to pitting.

9 The rate of penetration usuallydiminishes with time (Figure 3).The pit depth is typically limited to 0,5mm. If the metal gauge isadequate, perforation may not occur for a considerable time. The reasonfor the slowing reaction is the filling of the pit by hydrated aluminiumoxide, a gel like product that occupies +/- 20 times the volume of the hydrated oxide can also fill fairly tight crevices in fabricatedassemblies reducing their propensity to corrode. As only a small amountof metal is removed, the mechanical properties are not significantlyaffected. The product of CORROSION leaves a white bloom around the an engineering point of view, no major protective measures arenecessary.

10 However, for building prestige purposes where the initialbright surface finish has to be maintained, the material is usually anodisedor organic powder coated. A 6-monthly wash with a mild detergent,followed by thorough rinsing, is usually adequate to maintain the pitting normally diminishes with time in liquid media, the pitsdo not as readily stifle. Perforation may result. Prior to such an application,it is advisable to assess the corrosivity of the medium under operatingconditions to ALUMINIUM . This is accomplished by laboratory protection is always the best defense. The following steps could beemployed to prevent premature failure:zAnodisingzUsing alclad aluminiumzElectroplating, with nickelzPaintingzAvoiding stagnant conditions as this is contributory to the onset the use of steel, brass or copper fittings in, or in feeds, toliquid storage systems.