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Copyright © 2010 Midas Technologies Ltd. All rights reserved Technical Bulletin Telephone | +44 (0)1733 342600 Email | [email protected] Website | www.midastech.co.uk Galvanic Corrision Guide Let the Buyer have Faith What is Galvanic Corrosion? Galvanic corrosion is the corrosion that results when two dissimilar metals with different potentials are placed in electrical contact in an electrolyte or other conductable fluid or media. A difference in electrical potential exists between the different metals and serves as the driving force for electrical current to flow through the fluid or electrolyte. This current results in corrosion of one of the metals. The larger the potential difference, the greater the probability of galvanic corrosion. Galvanic corrosion only causes deterioration of one of the metals. The less resistant, active metal becomes the anodic corrosion site. The stronger, more noble metal is cathodic and protected. Galvanic corrosion potential is a measure of how dissimilar metals will corrode when placed against each other in an assembly. Metals close to one another on the chart generally do not have a strong effect on one another, but the farther apart any two metals are separated, the stronger the corroding effect on the one higher in the table. The table (left) that follows lists the potential differences for various metals in water with the table right indicating electrode potential. The order of the series can change for different electrolytes (for example, different pH, ions in solution, stagnant media, aerated or non aerated). Often when design requires that dissimilar metals come in contact, the galvanic compatibility is managed by finishes, isolation washers and plating. The finishing and plating selected facilitate the dissimilar materials being in contact and protect the base materials from corrosion. For harsh environments, such as outdoors, high humidity, and salt environments fall into this category. Typically there should be not more than 0.15 V difference in the “Anodic Index”. For normal environments, such as storage in warehouses or non-temperature and humidity controlled environments. Typically there should not be more than 0.25 V difference in the “Anodic Index”. For controlled environments, such that are temperature and humidity controlled, 0.50 V can be tolerated. Caution should be maintained when deciding for this application as humidity and temperature do vary from regions. If a noble metal like stainless steel has a large surface area in contact with the electrolyte while the sacrificial metal (such as galvanised steel) has a very small surface area in contact with the electrolyte, then the stainless steel will generate a large corrosion current which will be concentrated on a small area of sacrificial metal. The galvanised steel will corrode quickly ? first the zinc then the underlying steel ? and so galvanised fasteners in stainless steel are not acceptable. However, a stainless screw in galvanised steel is frequently used although a mound of zinc corrosion product will accumulate around the fastener. This is because the ratio of wetted noble fastener in an active metal might change from a 1:50 ratio to 1:1 during drying after a rainstorm. If contaminants are significant this means that avoiding dissimilar metal pairs may be a preferred option to prevent galvanic attack. As a rule of thumb, if the wetted area of the corroding metal is 10 times the wetted area of the noble metal, then galvanic effects are not serious although the larger the ratio the less the effect.

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Galvanic Corrosion and Prevention

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Page 1: Galvanic Corrosion Guide

Copyright © 2010 Midas Technologies Ltd. All rights reserved

Technical BulletinTelephone | +44 (0)1733 342600

Email | [email protected] Website | www.midastech.co.uk

Galvanic Corrision Guide Let the Buyer have Faith

What is Galvanic Corrosion?

Galvanic corrosion is the corrosion that results when two dissimilar metals with different potentials are placed in electrical contact in an electrolyte or other conductable fluid or media. A difference in electrical potential exists between the different metals and serves as the driving force for electrical current to flow through the fluid or electrolyte. This current results in corrosion of one of the metals. The larger the potential difference, the greater the probability of galvanic corrosion.

Galvanic corrosion only causes deterioration of one of the metals. The less resistant, active metal becomes the anodic corrosion site. The stronger, more noble metal is cathodic and protected. Galvanic corrosion potential is a measure of how dissimilar metals will corrode when placed against each other in an assembly. Metals close to one another on the chart generally do not have a strong effect on one another, but the farther apart any two metals are separated, the stronger the corroding effect on the one higher in the table.

The table (left) that follows lists the potential differences for various metals in water with the table right indicating electrode potential. The order of the series can change for different electrolytes (for example, different pH, ions in solution, stagnant media, aerated or non aerated). Often when design requires that dissimilar metals come in contact, the galvanic compatibility is managed by finishes, isolation washers and plating. The finishing and plating selected facilitate the dissimilar materials being in contact and protect the base materials from corrosion.

For harsh environments, such as outdoors, high humidity, and salt environments fall into this category.Typically there should be not more than 0.15 V difference in the “Anodic Index”. For normal environments, such as storage in warehouses or non-temperature and humidity controlled environments. Typically there should not be more than 0.25 V difference in the “Anodic Index”.

For controlled environments, such that are temperature and humidity controlled, 0.50 V can be tolerated.Caution should be maintained when deciding for this application as humidity and temperature do vary from regions.

If a noble metal like stainless steel has a large surface area in contact with the electrolyte while the sacrificial metal (such as galvanised steel) has a very small surface area in contact with the electrolyte, then the stainless steel will generate a large corrosion current which will be concentrated on a small area of sacrificial metal. The galvanised steel will corrode quickly ? first the zinc then the underlying steel ? and so galvanised fasteners in stainless steel are not acceptable.

However, a stainless screw in galvanised steel is frequently used although a mound of zinc corrosion product will accumulate around the fastener. This is because the ratio of wetted noble fastener in an active metal might change from a 1:50 ratio to 1:1 during drying after a rainstorm. If contaminants are significant this means that avoiding dissimilar metal pairs may be a preferred option to prevent galvanic attack.

As a rule of thumb, if the wetted area of the corroding metal is 10 times the wetted area of the noble metal, then galvanic effects are not serious although the larger the ratio the less the effect.

Page 2: Galvanic Corrosion Guide

Copyright © 2010 Midas Technologies Ltd. All rights reserved

Galvanic Corrision Guide Let the Buyer have Faith

Continued

Page 2

Galvanic Corrosion Chart

Magnesium AlloysZincBerylliumAluminum 1100, 3003, 3004, 5052, 6053CadmiumAluminum 2017, 2024, 2117Mild Steel 1018, Wrought IronHSLA Steel, Cast IronChrome Iron (active)430 Stainless (active)302, 303, 321, 347, 410, 416 Stainless Steel (active)Ni-Resist316, 317 Stainless (active)Carpenter 20Cb-3 Stainless (active)Aluminum Bronze (CA687)Hastelloy C(active) Inconel 625(active) Titanium (active)Lead/Tin SolderLeadTinInconel 600 (active)Nickel (active)60% Ni 15% Cr (active)80% Ni 20% Cr (active)Hastelloy B (active)Naval Brass (CA464), Yellow Brass (CA268)Red Brass (CA230), Admiralty Brass (CA443)Copper (CA102)Manganese Bronze(CA675), Tin Bronze(CA903, 905)410, 416 Stainless(passive) Phosphor Bronze(CA521, 524)Silicon Bronze (CA651, 655)Nickel Silver (CA 732, 735, 745, 752, 754, 757, 765, 770, 794Cupro Nickel 90-10Cupro Nickel 80-20430 Stainless (passive)Cupro Nickel 70-30Nickel Aluminum Bronze (CA630, 632)Monel 400, K500Silver SolderNickel (passive)60% Ni 15% Cr (passive)Iconel 600 (passive)80% Ni 20% Cr (passive)Chrome Iron (passive)302, 303, 304, 321, 347 Stainless (passive)316, 317 Stainless (passive)Carpenter 20Cb-3 Stainless (passive), Incoloy 825 (passive)SilverTitanium (passive), Hastelloy C & C276 (passive)GraphiteZirconiumGoldPlatinum

Magnesium Anodic

(least noble)

Corroded

Direction

of attack

Cathodic

(most noble)

Protected

Electrode Potential at 77 F (25 C)Anodic end (this is where the corrosion occurs)Element Standard Electrode Potential (Volts)

Lithium -3.045Potassium -2.920Sodium -2.712Magnesium -2.340Beryllium -1.700Aluminum -1.670Manganese -1.050Zinc -0.762Chromium -0.744Iron; Mild Steel -0.440Cadmium -0.402Yellow Brass -0.35050-50 Tin-Lead Solder -0.325Cobalt -0.277Nickel -0.250Tin -0.136Lead -0.126Hydrogen reference electrode 0.000Titanium +0.055Copper +0.340Mercury +0.789Silver +0.799Carbon +0.810Platinum +1.200Gold +1.420Graphite +2.250Cathodic end, passive (no corrosion here)

Page 3: Galvanic Corrosion Guide

Copyright © 2010 Midas Technologies Ltd. All rights reserved

Galvanic Corrision Guide Let the Buyer have Faith

Continued

Page 3

Stainless steel has an effective passive film so the available corrosion current is quite low. If the behaviour of a copper/steel and a stainless steel/steel couple is compared, the copper/steel coupling is a more significant galvanic problem despite the similar potential separation of 0.35 volts.

Examples of acceptable galvanic pairs include:Galvanised steel pipe hangers are used to hang stainless steel piping externally around chemical plants. The surface area ratio is bad with large area of stainless steel to small area of active zinc/steel but the rainwater is usually of quite low conductivity and 20 year service life is normal.

In the water industries, galling between stainless steel threads and nuts has been avoided by using aluminium bronze nuts on stainless steel studs or bolts. Although aluminium bronze is more active than stainless steel, the conductivity of the water, and hence the corrosion rate, is generally quite low. The nuts will require replacement but only at times of major overhaul.

One unacceptable case was a gasket with a carbon black loading so high it was conductive and caused severe galvanic attack of a 316 stainless lug. Graphite gaskets have caused similar problems.

The most severe conditions are in swimming pool / leisure club environments and serious incidents have occurred to the extent of roofs collapsing, applications for these should be properly documented and authorised.

Useless information or not?

A “lasagna cell” or “lasagna battery” is accidentally produced when salty food such as lasagna is stored in a steel baking pan and is covered with aluminum foil. After a few hours the foil develops small holes where it touches the lasagna, and the food surface becomes covered with small spots composed of corroded aluminum.

This metal corrosion occurs because whenever two metal sheets composed of differing metals are placed into contact with an electrolyte, the two metals act as electrodes, and an electrolytic cell or battery is formed. In this case, the two terminals of the battery are connected together. Because the aluminum foil touches the steel, this battery is shorted out, a significant electric current appears, and rapid chemical reactions take place on the surfaces of the metal in contact with the electrolyte. In a steel/salt/aluminum battery, the aluminum is higher on the electrochemical series, so the solid aluminum turns into dissolved ions and the metal experiences galvanic corrosion. If tomato ketchup was added to the top of the lasagna this would accelerate the process as the ketchup is highly acidic this would turn the aluminium foil into a grey mush on top of the lasagna.

Page 4: Galvanic Corrosion Guide

Copyright © 2010 Midas Technologies Ltd. All rights reserved

Our only Limitation is your Imagination

Midas Technologies (GB) Ltd, Midas House, Roundhouse Close, Eastern Industry, Peterborough, PE1 5TA Telephone | +44 (0)1733 342600 Facsimile | +44 (0)1733 346672 Email | [email protected]

Midas Technologies have provided this information in good faith based on their experience, sample test and research and development, together with freely available from other resources. We will not be responsible for any actions taken in respect of the information herein unless we are under contracts for the works. A compreensive Quality Assurance system is required from design and procurement of materials to installation.We recommend the use of ISO 9000:2000 registered firms.

The technical recommendations contained in this publication are necessarily of a general nature and should not be relied on for specific applications without first securing competent advice. Whilst Midas Technologies has taken all resonable steps to ensure the information contained herein is accurate and current it does not warrant the accuracy or completeness of the information and does not accept liability for errors or omissions.

Corrision Class 0Indoors with relative Humidity under 60%.Very Low Corrosion Risk

Corrision Class 1Indoors in non-heated, well ventilated room.Low Corrosion Risk.

Corrision Class 2Indoors with fluctuating temperature and humidity.Outdoor in inland climates, far from the sea and industry.Medium Corrosion Risk.

Corrision Class 3In densely populated areas or near industrial areas.In the vicinity of open water and near the coast.High Corrosion Risk

Corrision Class 4Constant, high humidity. Near industry manufacturing or utilizing chemicals.Very High Corrosion Risk.

With careful consideration during the design process galvanic corrosion can be avoided, be specific in pre treatments and final coatings such as degreasing, shot or glass bead blasting, chromate pre treatment, galvanising, zinc plating, polyester powder coating etc.