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Protection of Bimetallic Junctions

Protection of Bimetallic Junctions By A. G. Wilkie, A.R.I.C. IT is clear that when making plans for bimetallic junctions between the body Metals and other materials re­ and chassis of the vehicle. construction it is necessary to con­ quire a satisfactory life or service sider the materials being used from Much can be done in the methods under practical conditions and a corrosion aspect. In particular it is of fabrication to minimise the risk of to ensure this a knowledge of the bimetallic corrosion such as avoidance essential to watch for contact of dis­ similar metals and note their environ­ developments in the field of anti- of crevices and similar moisture traps. ment, since a galvanic cell may be Relative areas of the two metals should corrosion is necessary. The formed which could give rise to rapid be carefully watched as it is highly assessment of the suitability of and serious degradation of one of the desirable that the exposed area of the materials to be used in a par­ members. This aspect has been anodic metal should be as large as ticular structure presents cor­ possible in relation to the cathodic appreciated to a very great extent of recent years in the use of light alloys, metal in order to spread the anodic rosion as a serious and related since it was discovered that under attack around the areas of contact. problem. moist conditions some preventive Further precautions are, however, measure is essential if attachment is to essential in all cases where the bi­ be made to an iron or steel member. aluminium will give rise to attack of metallic junction is subjected to mois­ ture. An insertion medium is here the aluminium. Electrochemical series required which will keep out moisture Atmospheric exposures inland are and other corrosive influences whilst far less conducive than marine ex­ By reference to the electrochemical maintaining an extremely high elec­ posures to galvanic attack, because an series in which the zero of potential is trical resistance between the two metal electrolyte is present in saline con­ chosen arbitrarily, being that of a faces. It is also desirable that the normal hydrogen electrode, it will be ditions which will readily conduct the insertion should incorporate an in­ seen that the metals in equilibrium flow of electrical current between the hibitor as an additional means of pro­ with solutions of their salts possess members. Under marine conditions, tection, as any product will depend widely differing electrode potentials. aluminium is anodic to all common almost as much on its application as structural metals apart from mag­ At the top end of the electrochemical on its general efficacy. nesium and zinc alloys, although there series will be found the 'noble ' metals is little corrosion with stainless steels, such as platinum and silver, whilst at A proprietary impregnated and the lower end are found the strongly particularly if the steel is unpitted and coated tape based on petrolatum with electronegative, 'less noble' metals the aluminium surface is relatively a metallic chromate inhibitor is avail­ which are more easily reduced to their large. able and proving of great value in the salts. industries mentioned. This material, Anomalies in the electrochemical whilst possessing the above properties, series are found to arise in practice ELECTROCHEMICAL SERIES effectively 'wets' the metal sur­ due to factors such as the natural Electrode faces and maintains its protection Metal potential resistant film of oxide covering alu­ (volts) indefinitely, remaining permanently minium. In such a situation, zinc Platinum ... + 1.20 plastic and waterproof under extremely becomes anodic to aluminium although Silver ... +0.80 adverse conditions. higher in the table, and similarly cad­ Copper ... +0.35 More noble metals Hydrogen ... 0.00 mium becomes anodic to iron and Lead ... -0.1 3 Less noble metals therefore cadmium plating protects Tin ... ... -0.1 4 the latter sacrificially (whereas the New Birmingham representative Nickel ... -0.2 5 reverse is to be expected from the Cadmium ... -0.4 0 Detel Products Ltd., manufacturers relative positions in the tables). Iron (ferrous) -0.4 4 of protective coatings against acid, Zinc ... -0.7 6 alkali and sea water, have announced Aluminium ... -1.6 7 Bimetallic junctions in practice the appointment of H. R. Burrows as Magnesium ... -2.3 7 Sodium ... -2.7 1 their technical sales representative for Bimetallic junctions met in practice Birmingham and the Midlands. Practical significance of the electro­ are principally those between light­ chemical series lies in the fact that the weight aluminium or magnesium alloys Jenolizing Co. appointment relative danger of coupling any two and iron or steel members. Aluminium metals together without protection can alloys are being used in ever-increasing J. Rutter has been appointed a tech­ be estimated; this can be done quali­ quantities by the shipbuilding industry nical representative of the Jenolizing tatively by looking for the distance to effect weight saving in construction, Co. Ltd., manufacturers of phosphat- apart in the table, i.e. what difference and bimetallic junctions requiring ing processes. He is at present under­ in potential lies between the two protection are thus found where going intensive training in their Rath- metals. The metal lowest in the table aluminium wheelhouses, deckhouses bone Street, W.1, laboratories. Pre­ will normally be the anode and there­ and other superstructures are built on viously he was preparing and con­ fore the one to be attacked. Thus, to steel decks. Motor transport con­ trolling painting specifications for the iron attached to copper will normally struction developments also are de­ Commonwealth Government in Syd­ lead to severe attack of the iron, par­ pending more and more on lightweight ney, Australia, and he brings to his ticularly if the flow of liquid is from alloys, striving to obtain improved new appointment a wide knowledge copper to iron in the case of water size/weight ratios whilst maintaining of paint technology based on 20 years' pipes, and, similarly, iron attached to strength and stability, which leads to experience in the paint industry. 156 CORROSION TECHNOLOGY, May 1955 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Anti-Corrosion Methods and Materials Emerald Publishing

Protection of Bimetallic Junctions

Wilkie, A.G.
Anti-Corrosion Methods and Materials , Volume 2 (5): 1 – May 1, 1955
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Publisher
Emerald Publishing
Copyright
Copyright © Emerald Group Publishing Limited
ISSN
0003-5599
DOI
10.1108/eb019058
Publisher site
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Abstract

By A. G. Wilkie, A.R.I.C. IT is clear that when making plans for bimetallic junctions between the body Metals and other materials re­ and chassis of the vehicle. construction it is necessary to con­ quire a satisfactory life or service sider the materials being used from Much can be done in the methods under practical conditions and a corrosion aspect. In particular it is of fabrication to minimise the risk of to ensure this a knowledge of the bimetallic corrosion such as avoidance essential to watch for contact of dis­ similar metals and note their environ­ developments in the field of anti- of crevices and similar moisture traps. ment, since a galvanic cell may be Relative areas of the two metals should corrosion is necessary. The formed which could give rise to rapid be carefully watched as it is highly assessment of the suitability of and serious degradation of one of the desirable that the exposed area of the materials to be used in a par­ members. This aspect has been anodic metal should be as large as ticular structure presents cor­ possible in relation to the cathodic appreciated to a very great extent of recent years in the use of light alloys, metal in order to spread the anodic rosion as a serious and related since it was discovered that under attack around the areas of contact. problem. moist conditions some preventive Further precautions are, however, measure is essential if attachment is to essential in all cases where the bi­ be made to an iron or steel member. aluminium will give rise to attack of metallic junction is subjected to mois­ ture. An insertion medium is here the aluminium. Electrochemical series required which will keep out moisture Atmospheric exposures inland are and other corrosive influences whilst far less conducive than marine ex­ By reference to the electrochemical maintaining an extremely high elec­ posures to galvanic attack, because an series in which the zero of potential is trical resistance between the two metal electrolyte is present in saline con­ chosen arbitrarily, being that of a faces. It is also desirable that the normal hydrogen electrode, it will be ditions which will readily conduct the insertion should incorporate an in­ seen that the metals in equilibrium flow of electrical current between the hibitor as an additional means of pro­ with solutions of their salts possess members. Under marine conditions, tection, as any product will depend widely differing electrode potentials. aluminium is anodic to all common almost as much on its application as structural metals apart from mag­ At the top end of the electrochemical on its general efficacy. nesium and zinc alloys, although there series will be found the 'noble ' metals is little corrosion with stainless steels, such as platinum and silver, whilst at A proprietary impregnated and the lower end are found the strongly particularly if the steel is unpitted and coated tape based on petrolatum with electronegative, 'less noble' metals the aluminium surface is relatively a metallic chromate inhibitor is avail­ which are more easily reduced to their large. able and proving of great value in the salts. industries mentioned. This material, Anomalies in the electrochemical whilst possessing the above properties, series are found to arise in practice ELECTROCHEMICAL SERIES effectively 'wets' the metal sur­ due to factors such as the natural Electrode faces and maintains its protection Metal potential resistant film of oxide covering alu­ (volts) indefinitely, remaining permanently minium. In such a situation, zinc Platinum ... + 1.20 plastic and waterproof under extremely becomes anodic to aluminium although Silver ... +0.80 adverse conditions. higher in the table, and similarly cad­ Copper ... +0.35 More noble metals Hydrogen ... 0.00 mium becomes anodic to iron and Lead ... -0.1 3 Less noble metals therefore cadmium plating protects Tin ... ... -0.1 4 the latter sacrificially (whereas the New Birmingham representative Nickel ... -0.2 5 reverse is to be expected from the Cadmium ... -0.4 0 Detel Products Ltd., manufacturers relative positions in the tables). Iron (ferrous) -0.4 4 of protective coatings against acid, Zinc ... -0.7 6 alkali and sea water, have announced Aluminium ... -1.6 7 Bimetallic junctions in practice the appointment of H. R. Burrows as Magnesium ... -2.3 7 Sodium ... -2.7 1 their technical sales representative for Bimetallic junctions met in practice Birmingham and the Midlands. Practical significance of the electro­ are principally those between light­ chemical series lies in the fact that the weight aluminium or magnesium alloys Jenolizing Co. appointment relative danger of coupling any two and iron or steel members. Aluminium metals together without protection can alloys are being used in ever-increasing J. Rutter has been appointed a tech­ be estimated; this can be done quali­ quantities by the shipbuilding industry nical representative of the Jenolizing tatively by looking for the distance to effect weight saving in construction, Co. Ltd., manufacturers of phosphat- apart in the table, i.e. what difference and bimetallic junctions requiring ing processes. He is at present under­ in potential lies between the two protection are thus found where going intensive training in their Rath- metals. The metal lowest in the table aluminium wheelhouses, deckhouses bone Street, W.1, laboratories. Pre­ will normally be the anode and there­ and other superstructures are built on viously he was preparing and con­ fore the one to be attacked. Thus, to steel decks. Motor transport con­ trolling painting specifications for the iron attached to copper will normally struction developments also are de­ Commonwealth Government in Syd­ lead to severe attack of the iron, par­ pending more and more on lightweight ney, Australia, and he brings to his ticularly if the flow of liquid is from alloys, striving to obtain improved new appointment a wide knowledge copper to iron in the case of water size/weight ratios whilst maintaining of paint technology based on 20 years' pipes, and, similarly, iron attached to strength and stability, which leads to experience in the paint industry. 156 CORROSION TECHNOLOGY, May 1955

Journal

Anti-Corrosion Methods and Materials – Emerald Publishing

Published: May 1, 1955

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