Life is full of options - and anodizing coloring is full of options, too!
In my 25 years as a consultant in the anodizing industry, I realized that about 70 % of my consulting clients were not anodizers but the people who had to “deal with” anodizers. Perhaps you have had also frustrating experience ordering anodized parts and not receive what you wanted?
You might think that your anodizer was just not good at their job. Or that anodizers will always try to keep their costs as low as possible and thereby sacrificing the quality of your aluminum product. But that does not do the problem justice. In my experience, the main reason for errors and frustrations is ineffective communication. Yip, just like in any relationship, the relation to your anodizer will improve tremendously by learning how to communicate better.
To make your next phone call with your anodizer much more successful, here is a checklist with all the aspects of the anodizing process you should talk about.
This checklist is a good start to improve the communication with your anodizer and to receive the correct anodized parts. If you would like step up your game even more and become more confident in asking the right questions, join my new live online training: “Working with your anodizing supplier.”
The course consists of four workshops, each treating an independent subject. You can also attend single workshop days. This interactive live online event is held on Zoom in 4 x 2,5 hours sessions. We will record the presentation and make them available to you after the workshop. The dates of our next courses are:
WORKSHOP SERIES IN APRIL: 7, 14, 21, and 28 April 2021 (03.00 - 5.30 PM CEST)
WORKSHOP SERIES IN MAY: 5, 12, 19, and 26 May 2021 (03.00 - 5.30 PM CEST)
Looking forward to seeing you at AnodizingSchool!
Sunny regards,
Anne Deacon Juhl
P.S. If you would like to learn some anodizing basics, have a look at this blog post in which I give an easy to understand introduction.
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A question from a customer:
We have investigated the influence of the electrolyte temperature in our anodizing tank - Anodizing at a low and a warm temperature, current controlled and in our standard anodizing electrolyte.
The voltage at the higher temperature runs with a lower value than the voltage with the lower temperature and at the same time the voltage stays constant for the duration of the anodizing process.
At the lower temperature the voltage keeps rising until we turn of the power.
Do you have an explanation of why the voltage at the higher temperature stays constant, although the layer thickness is identical with both temperatures (with identical current settings and anodising time)?
Answer:
Higher temperature in the anodizing electrolyte leads to higher conductivity, which means lower voltage for same current and time.
So your Vhigh temp is lower than your Vlow temp
Then
Ohms law gives you V = R x I
Vhigh temp = Rhigh temp x I
I =
same for the two temperatures
The
total resistance of the electrical circuit consists of several resistances:
High
temp:
Rhigh temp = Relec, high temp + Rthickness,
high temp + …..
Low
temp:
Rlow temp = Relec, low temp + Rthickness,
high temp + …..
Lower temperature gives lower conductivity, so Relec,
low temp ˃
Relec, high temp
which is the reason for a higher Vlow temp
Higher temperature of the electrolyte will lead to faster chemical attack of the formed aluminum oxide changing the structure of the formed oxide and the resistance Rthickness, high temp
This will lead to an equilibrium thickness, where formation rate of oxide = dissolution rate of oxide leading to a constant voltage.
Lower temperature of the electrolyte will lead to a higher V0, low temp than V0, high temp for same current I.
The coating weight of the oxide layer is higher when formed at lower temperature - more compact and by this Rthickness, low temp will be higher than Rthickness, high temp.
This is the reason for a continues increase in Vlow temp during the process time but eventually you should see a steady voltage here too.
If you are curious and want to know more about temperature, voltage and other parameters when anodizing in sulfuric acid you should sign up for the first and only online anodizing course!
| | 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* | |
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.
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.
| Register by July 14th and save $100 The anodizing workshop will be held at Holiday Inn Express San Diego Old Town Make sure to let them know you will be attending the Surface Finishing Academy's Introduction to Anodizing Course to get the best possible rate. Holiday Inn Express Old Town |
If you simply just want to know more about anodizing, or why some parts are rejected and others not, or enjoy a great network opportunity with other people who love to talk about anodizing, then join us by clicking here.
The main use of Chromic Acid Anodizing is due to the fact that residues from the chromic acid trapped in parts that are difficult to rinse does not lead to corrosion. Another important feature is the fact that Type I coatings keep the aluminum materials fatigue strength and the very thin layer makes a minimal dimensional change.
Alloys are not allowed to contain more than 5% copper or 7% silicon, and total alloying element must not exceed 7.5% according to MIL-A-8625F. The alloys should be in one of the following temper before anodizing, T4, T6 or T73.
The electrolyte should consist of 50 - 100 g/L chromic acid and with a temperature of 95 - 105F. The purity of the chromic acid should not be less than 99.5% CrO3. Chloride is the worst contamination for the electrolyte and shouldn´t exceed more than 20 g/L. Chloride present in the electrolyte causes etching of the aluminum.
The hexavalent chromium content, the free chromic acid, decreases during the process and the trivalent chromium and aluminum increase.
Most of the Chromic Acid Anodizing is processed at 40 V, the low voltage is only used for special alloys which are difficult to handle at the higher voltage, as e.g. 2014 and 7075. Ramping is essential is this type of anodizing.
An easy way to remember the ramping time is the following;
For the all the alloys using 40 V, use 40 minutes to increase the voltage to 40V and then anodize for another 30 minutes. For the alloys using 20V, the same pattern can be used, use 20 minutes to get to the 20V and then spend another 30 minutes at that voltage.
The two different voltages used for Type I and Type IB create a slighty difference in the oxide film formed which can be seen in SEM images below of the surfaces.
Type IB, 22Volt
Type I, 40 V
The specifications for the performance of the coatings are a little different than for hard anodizing but the same procedures should be used. The difference is the weight of the formed coating which should be min 200 mg/ft2 and the coating should pass 336 hour in a salt spray test for Type I coatings.
It has to be mentioned that hexavalent (CrVI) compounds, often called hexavalent chromium, exist in several forms. Hexavalent chromium is recognized as a human carcinogen via inhalation. For more information check out, United States Department of Labor or The European Union directive, Directive 2002/95/EC.
If you find this article useful and you would like to know more please contact me blog@aluconsult.com
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