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.


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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.
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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.
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Happy New Year

Dear Anodizing World readers2011 is already rolling with the possibility of new ideas, new goals and new questions.With a hope of a prosperous 2011 I look forward to write a lot of interesting posts about aluminium and anodizing this year.This year wil… Continua a leggere

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A question about corrosion of aluminum in contact with stainless steel

I always appreciate when my readers contact me for more in-depth information regarding a specific issue. This time the question was regarding galvanic corrosion between aluminum and stainless steel.

The earlier post the question came from is the post, Corrosion between anodized aluminum and steel.

The question was:

I´m still a bit confused on the effect anodizing has to this corrosion problem. You stated that it can be superior choice but also make it worse. How will I know?

Below you will find my answer:

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, but because of the natural formed oxide layer Aluminum can be a called a passive metal. So Aluminum behaves as a very stable metal, especially in oxidizing media such as air, water, etc.. This natural formed oxide layer differs in density compare to the underlying aluminum, which makes the aluminum oxide less likely to crack when deformed. The dissolution rate of aluminum oxide depends on the pH value, see figure below.

The corrosion rate (dissolution rate of the aluminum oxide) is not solely dependent on the pH but also what kind of acid or alkaline solution we are talking about. Sodium hydroxide at 0.1 g/l is 25 times higher than in an ammonia solution at 500 g/l. For the acids solutions of hydrochloric acid or hydrofluoric acid are much more aggressive than solutions of acetic acid. It is though very important to recognize the different slopes of the curve depending on which side on the pH scale the aluminum is exposed too. High pH has much higher corrosion rate than for a low pH.

So what I am saying is, take a careful look at your environment, if you are out of the pH range 4.5 – 8.5 you should immediately be aware of a possible corrosion issue. This is the same whether you have an anodized surface or not. The protective oxide film will not be protective anymore, leaving a part smaller or bigger part of the aluminum unprotected.

If a very small area of the protective film has been destroyed, exposing a small anode area, as shown it the picture in the post, due to cracks or a scratch or something else then you will know that you have a corrosion problem.

This will lead to a small anodic area (un-protective aluminum) relative to the cathode area (the stainless steel) and this should be avoided. The larger the relative anode area, the lower the galvanic current density on the anode, the lesser the attack.

A practical illustration of what I am saying is on anodized aluminum frames in windows, especially in salty environments. Some have stainless steel clips riveted to them destroying the anodic oxide film, in other places there are normal carbon steel bolts through the frames with no destruction. There is zero evidence of corrosion between the steel bolts and the anodized aluminum frame. In the areas of the rivets the anodized aluminum frame is completely and totally eaten away underneath the stainless steel clips. Outside this region there is no corrosion.

If you have a specific problem, I would happy to help you as a consultant . Take a look at one of my products and let me know if you would be interested.

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Hardness versus Wear resistance

The first post about the hardness of the aluminum oxide formed by the hard anodizing process described the various types of hardness test, Vickers, Knoops, Brinell and Rockwell.


This second one will give an idea of the differences between the hardness of the hard anodized coating (hard anodizing) and the wear resistance of the hard anodize anodic process. This text will also have referrals to presentations at the International Hardanodizing Association´s symposium in September 2010 in Las Vegas.

Aluminum oxide is almost as hard as diamond (1200 HV or more) but in general too thin to increase the hardness of the aluminum metal itself. It will not protect against strong pressure but it will resist surface scratches and therefore protect the overall appearance of the surface.

The hardness of the aluminum oxide layer formed by anodizing, increases by decrease in temperature of the electrolyte and in the acid concentration. The hardness is also increased by an increase in the homogeneity of the microstructure and by an increase in the current density used to form the aluminum oxide film.

Prof. Allan Matthews of the University of Sheffield, England pointed out in a presentation that “Wear = constant load/hardness is a commonly accepted relationship. However this equation is nonsense, because it ignores the many different types of wear, such as impact, fretting, abrasion, friction sliding and others.”

In fact, the Elastic modulus is also very influential, and the ratio of hardness (H) to modulus (E) gives a better indication of wear resistance than either alone.

If the hardness is too high, the coating is susceptible to cracking. However, ductility allows a coating to accommodate deformation. When the ration H/E is high, then wear resistance is good.

Mr. Leonid Lerner from Sanford Process Corp., US showed in his presentation at the IHAA symposium a great slide of the two different directions which we expose the oxide layer for external stresses depending on if we test or use it in normal applications.

When testing the aluminum oxide film formed by the hard anodizing process it is normally done on a cross, shown in Image A and is explained more in the first post about how to define the hardness of aluminum oxide formed by hard anodizing.

This leads to a stress horizontal and perpendicular to the hexagonal oxide cell structure (Cross-sectional View, see right bottom of the slide).

Whereas the mechanical stresses in normal applications will be vertical and perpendicular to the hexagonal oxide cell structure (Top View, see left bottom of the slide).

So even though the hardness of the aluminum oxide film itself is very hard, it is way to thin to increase the hardness of the aluminum metal itself.

The hardness of the aluminum material is most often proportional to the abrasive wear resistanc but as explained above,the hardness of the aluminum oxide film formed by hard anodizing will not always be proportional to abrasive wear resistance.

Maximum abrasion resistance of the aluminum oxide is found on pure aluminum and aluminum-magnesium alloys for the same hard anodizing process parameters.

Sealing decreases the wear resistance of oxide film formed by hard anodizing up to 50 – 70 % of the unsealed value.

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