Galvanic corrosion = Bi-metallic corrosion on Aluminum

One of my most read articles on Anodizing World is the one about galvanic corrosion, therefore I have decided to dwell a little bit more about galvanic corrosion on aluminum.

When two dissimilar metals are in direct contact in a conducting liquid, experience shows that one of the two will corrode - hereby the name - Bi-metallic Corrosion or also called Galvanic Corrosion.

If two metals, e.g. aluminum and copper, are placed in sea water both metals will start to corrode. If the two metals are connect with a wire a current will flow, and aluminum will corrode faster and the copper will stop corroding. This is like a sacrificial anode of zinc on a boat, but here the anode is the aluminum.

Aluminum is a reactive (un-noble) metal compared to most of the metals used. Aluminum will therefore almost always be the anode, the part which corrodes, in contact with other metals.

The two main factors are the severity of the environment and the potential difference between the two materials, e.g. aluminum and copper.

The more aggressive the environment and the larger the difference in potential between the two metals are the more sever is the corrosion attack.

The bible "Surface Treatment and Finishing of Aluminum and its Alloys" by S.Wernick, R. Pinner and P.G. Sheasby shows the following table.

General guide to galvanic influence of various metals on aluminum.

Metal
Comment
Cadmium
Slight effect
Chromium
Small to negligible
Copper and copper alloys 
Severe
Graphite
Severe
Lead
Negligible except in severe marine environments
Stainless steel
Negligible except in severe marine environments
Steel and Iron                               
Slight except in severe environments
Tin
Negligible except in salt solutions
Titanium
Negligible except in severe marine environments
Galvanised steel
No effect until zinc coating is destroyed

If you find this article useful and you would like to know more please contact me blog@aluconsult.com
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Troubleshooting Aluminum surface issues!

Send me your surface finishing issue per. email, preferably with a photo, then I will ask detailed questions which you answer before the call, during the call we will clear out your questions together, coming up with ideas, new opportunities and results. After this call if you have any follow-up questions about the topic, you can sent an email, which I answer within in 48 hours.

Your investment?


An e-mail and one hour, and $495. Money you've earned into multiples when you can reduce your time used on this specific issue, and know what to expect of your product and what your requirements are.


Payment is easily done by PayPal using any major credit card.


__________________________________________________

Troubleshooting Aluminum Surface Issues!

Send me your surface finishing issue per. email, preferably with a photo, then I will ask detailed questions which you answer before the call, during the call we will clear out your questions together, coming up with ideas, new opportunities and results. After this call if you have any follow-up questions about the topic, you can send an email, which I answer within in 48 hours.

Your investment?

An e-mail and one hour, and $495. Money you've earned into multiples when you can reduce your time used on this specific issue, and know what to expect of your product and what your requirements are.


Payment is easily done by PayPal using any major credit card.

______________________________________________________

Anodizing Conference in September

In the middle of September I will attend the yearly Aluminum Anodizers Council conference in Denver.



The Aluminum Anodizing & Extrusion Summit is a combined conference of the Aluminum Anodizers Council (AAC) and the Aluminum Extruders Council (AEC). The co-located AAC Annual Anodizing Conference and AEC Management Conference offer tools, information and connections so aluminum professionals can make informed decisions.

The program consists of interesting papers such as update regarding the specification for anodized aluminum and its alloys - MIL-A-8625F by Mark Jozefowicz, Reliant Aluminum Products, LLC and News fro the Automotive industry by Gregg Peterson, Michigan Manufacturing Technology Center.

The more technical ones will include interesting news regarding coloring processes, light fastness, acid etch, environmental friendly bright dip finish and much more.

The AAC focus tracks are Recreational Applications Track, Color and Dye Technology and Technical Issues Track.

The AEC focus tracks are Promoting Aluminum Extrusion Track, Trade Issues Track and Management & Strategy Track.

If you find this article useful and you would like to know more please contact me blog@aluconsult.com __________________________________________________


Aluminum and construction

Often corrosion is seen as something that happens, when a material is exposed to the elements.

Corrosion however, happens very often much earlier, namely on the construction sites, where the materials are stored inappropriately and without thought for what each material can actually tolerate.

Anodized aluminum surfaces are being used more and more in construction due to the very beautiful and characteristic appearance of an anodized surface has. The metallic surface in conjunction with the ability to color and decorate aluminum surface provides a wealth of unique and innovative looks at buildings.






These beautiful surfaces hold for the vast majority of outdoor exposure, but often the damage has happened before due to accidental contamination or inadequate storage.

Aluminium surfaces have, with or without anodizing a resistance towards corrosion, as long as the pH remains in the neutral range - pH between 4.5 and 9.
If you find this article useful and you would like to know more please contact me adj@aluconsult.dk

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Anodizing for Aerospace - Article in Metal Finishing 2010

The oxide film formed by various anodizing processes is mechanically superior and produces a much higher corrosion- and abrasion-resistant layer than the chemical conversion coatings. The various processes all use an electrical current to form the oxide film. The current passes through an electrolyte in which aluminum is the anode, hence, the name “anodizing.” The nature of the electrolyte, the reaction produced and operation parameters determine the structure and properties of the formed oxide film.
This overview will provide a short explanation of the various anodizing processes used in the aerospace industry today.

THE VARIOUS ANODIZING PROCESSES

Many electrolytes have been tested, used and patented during the last century, leaving only a few as important industrial processes. According to the “bible” of anodizing, “The Surface Treatment and Finishing of Aluminum and its Alloys” by Wernick, Pinner and Sheasby, the three most important ones are chromic acid, sulfuric acid or oxalic acid.1 Acids as phosphoric acid and boric sulfuric acid mix are now used in the market for anodizing in the aerospace industry.
Chromic acid anodizing, or CAA, was the first commercial anodizing process patented in 1923 by Bengough and Stuart, followed closely by the first sulfuric acid anodizing (SAA) process patented in 1927.
The oxalic acid was introduced by the Japanese in the middle of the 1950’s. The main interest today is as an additional acid in hard coat anodizing, or HCA, to produce a harder coating faster than that obtained with a pure sulfuric acid electrolyte.
Phosphoric acid anodizing, or PAA, and boric sulfuric acid anodizing, BSAA, were both developed by the Boeing Company, the first one as a structural bonding surface and the other as a replacement for CAA for non-critical fatigue parts. The most commonly used anodizing process is the sulfuric acid anodizing process, but for the aerospace applications this picture looks a little different.
Chromic acid anodizing is mostly used for protection of critical structures with all kinds of joints. The corrosion resistance is excellent relative to the thickness of the coating, which normally lies in the range of 0.08 – 0.2 mil. The oxide film is softer and less porous than those formed by the other processes, and is formed without any significant fatigue loss of the material. The film is easily damaged, and the color is light opaque gray. When this film is sealed in a dichromate seal, a greenish color appears.
The process is voltage controlled with a ramping in the beginning of the process increasing up to 40 volts depending on the type specified. Two types are specified in the military specification MIL-A-8625F, type I and Type IB, whereas the first is conventional coatings produced by a voltage of around 40 volts and Type IB uses a voltage of 20 to 22 volts.
Sulfuric acid anodizing can be divided into two main uses, for Type II coatings and Type III coatings. Type II is primarily used for decorative or protective applications, whereas hard coat oxide films, Type III, are used for engineering applications, i.e., the aerospace industry.
MIL-A-8625F specifies the Type III coatings as those formed by treating aluminum and its alloys electrolytically to produce a uniform anodic coating. This gives a variety in the process operations procedures as long as a heavy, dense coating is produced.
The resultant hard film is very dependent on the aluminum alloy used.The first processes used higher current densities and lower temperatures of the electrolyte. These process parameters give some difficulties with higher copper alloys of the 2000 series—some of the favorite alloys for the aerospace industry. Therefore, a lot of work has been done to reduce these difficulties.3,4 Addition of oxalic acid to the sulfuric acid electrolyte has been one of the main modifications. Additionally, variation in electrolyte temperature and the use of different electrical sources and pulse methods have been developed.5,6,7
Phosphoric acid anodizing is basically used for structural adhesive bonding in high-humidity environments. This process is known as the Boeing Process and is carried out at 10-15 V. The formed oxide film has a greater durability under adverse conditions than film formed in chromic acid and sulfuric acid. One of the reasons for the great adhesive property is said to be due to the morphology of the oxide film, which should be a film of pores with whiskers or protrusions on the top surface of the formed film.
The last anodizing process mentioned is the new boric sulfuric acid. This is an alternative to the chromic acid electrolyte, which contains hexavalent chromium. Note: Hexavalent chromium is carcinogen and has to be phased out of metal finishing processes. Therefore, hexavalent chrome-free electrolytes are necessary. The formed oxide film from the boric sulfuric electrolyte has a paint adhesion that is equal, or superior, to the one formed on chromic acid. The process is voltage controlled and is ramped to 15 V. A seal in a hot dilute chromate solution is required to achieve satisfactory corrosion resistance.
The above processes are the basis of the anodizing we do in the aerospace industry today. It should be remembered that operating conditions might vary within a wide range, and that most of the specifications are general guidelines. Therefore, the most important part to remember is to define the performance criteria before choosing the right anodizing process.

REFERENCES

  1. Wernick, S., Pinner, R. and Sheasby, P.G., “The Surface Treatment and Finishing of Aluminum and its Alloys”, 5. Ed., Finishing Publications LTD., Teddington, Middlesex, England, 1987.
  2. Juhl, A. Deacon, “Hard Anodizing of Aerospace Aluminum Alloys”, Light Metal Age, June 2009.
  3. Lerner, L., Sanford Process Corporation, “Hard Anodizing of Aerospace Aluminum Alloy”, presented at IMFAIR09, 10-11 June, 2009, Royal Air Force Museum, Cosford, Shropshire UK.
  4. Schaedel, F., “Improving Anodize Wear and Corrosion Resistance by Combining Modified Electrolyte Chemistry with Advanced Waveform Pulse Ramp Technology”, AAC 17th Anodizing Conference & Exposition, October 28–30, 2008, San Francisco. 
  5. Munk, F., “State of the Art Hardcoat Anodizing Power Supplies”, IHAA, 9th Technical Symposium, Canada, Sept., 2002
  6. Juhl, A. Deacon, “Pulse Anodizing of Extruded and Cast Aluminium Alloys”, Ph.D. thesis, Inst. of Manufacturing Engineering, The Technical University of Denmark, July, 1999.
  7. Juhl, A. Deacon, “Why it Makes Sense to Upgrade to Pulse Anodizing”, Metal Finishing, July/August 2009.


If you find this article useful and you would like to know more please contact me blog@aluconsult.com 
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Exhibitor at the Aluminium 2016 - 11th World Trade Fair and Conference

For the second time Exhibitor at the Aluminium 2016 - 11th World Trade Fair and Conference

This time at the Danish Pavilion together with a lot of very interesting and specialised Danish Aluminium Companies.

Come by G10 50-60 during the next three days and have a talk with me or some of the other companies. 

There will be a lot of talk about pulse anodising, and other interesting surface treatments. We are introducing the new Aluminium Competence Center - Aluminium Hot Spot.

If you find this article useful and you would like to know more please contact me blog@aluconsult.com __________________________________________________

Back on track with new posts about my favorite subject - anodizing/anodising of aluminium

After working several years in the aerospace surface treatment area - learning a lot about chromic acid anodizing/anodising as a Supervisor and ME, I am back as a consultant to help around the world with my favorite subject - aluminium, anodizing/anodising and other types of surface treatments of aluminium. Anodizingworld.com will again send out short and important posts about aluminium, anodizing/anodising and surface treatment issues which can help you to get a better understanding and more ideas of how to use this magnificent material aluminium anodized/anodised is.

If you find this article useful and you would like to know more please contact me blog@aluconsult.com __________________________________________________