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 ajuh@teknologisk.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 __________________________________________________

White paper about the global need of knowing more about corrosion

Take a few minutes of your day and read this white paper about corrosion - not only corrosion on aluminum.

Extract from the foreword of the white paper.

The science of corrosion prevention and control is highly complex, exacerbated by the fact that corrosion takes many different forms and is affected by numerous outside factors. Corrosion professionals must understand the effects of environmental conditions such as soil resistivity, humidity, and exposure to salt water on various types of materials; the type of product to be processed, handled, or transported; required lifetime of the structure or component; proximity to corrosion-causing phenomena such as stray current from rail systems; appropriate mitigation methods; and other considerations before determining the specific corrosion problem and specifying an effective solution.

One of the figures shows severe pitting corrosion on aluminum and how temperature and aluminum alloy influence on the corrosion attack.
If you have a corrosion problem you need to solve on aluminum, I have the solution and you can get started immediately for the price of $495.

Interested send me an email adj@aluconsult.com


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

Aluminium 2000 congress in Bologna, Italy

Last week the 7th Aluminium Two Thousand International Congress took place in Bologna, Italy. Around 350 people attended from 25 different countries.

With the motto "Let´s build the future of the aluminum world together" the days were fully packed with experts from all around the world. The three day program with parallel sessions included analysis of the aluminum industry, interesting new developments within all different aspects of the aluminum industry; foundry, casting, extrusion, anodizing and painting, automation, architecture, transport industry, environmental protection and recycling, measuring, testing and quality techniques.

A lot of the presentations were about how to save money, which is probably understandable because this is a topic we all can relate to and want to hear more about.

So I was very honored to be asked to present a paper on the cost savings when changing from conventional DC anodizing to Pulse anodizing.

The paper and presentation showed that the ROI is less than a year for an Anodizing line to switch from conventional DC to Pulse anodizing - all because of the increase in productivity.

I am proud to say that the paper, among three others, was awarded "Most interesting presentation", at the congress.


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

Coefficient of Friction between Anodized Aluminum and Steel

First a short introduction to friction and the friction coefficient.

Friction is the force resisting when two parts are moved against each other. This can be between solid surfaces, fluid layers and/or material elements. The subject here is between two solid surfaces, also called dry sliding friction, no fluid in the sliding area.

The coefficient of friction is defined by the applied load between two parts, L, and the resultant friction force required to slide the two parts, F.

The Coefficient of Friction, µ, is given by
µ=F/L (the value is dimensionless).

The dry sliding friction coefficients vary a lot depending on the surfaces characteristic of the two parts. It is important to mentioned here that all friction coefficient values should be treated with caution because the value is very dependable of the environment and operating conditions.

The following table is taken from SIS Handbook, Aluminium, ed. 3, June 2003 and edited by me. The aluminum alloy used is not mentioned.

Against steel

Against its self

Hard anodizing

0,22

0,17

Anodizing

0,30

Hard anodizing with Teflon

0,14

0,11

Aluminum

0,61*


* from Wikipedia

The value of the friction coefficient has to be dependent on the uniformity and quality of the anodic layer formed. So therefore the value would be dependent of the aluminum alloy used because of the difference in quality of the anodic layer.

For any specific application the ideal method of determining the coefficient of friction is by trials.


As mentioned above sometimes there is a fluid layer involved which will immediately change the picture. This figure is taken from The English Surface Finishing Company, Poeton.





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