Crazing defects in the anodic layer formed by sulfuric acid anodizing is a well known phenomena. It is often readily seen by the naked eye as a kind of a spider web.
The anodic oxide layer will always have some cracks, so it is how to minimize these which is important. The crazing defects are usually always found on anodized aluminum which has been exposed to high temperatures or any considerable sudden changes in temperature.
The reason for this is the difference in thermal expansion for the aluminum material and the coating.
Anodic coating: 5 x 10-6/°C
Aluminium: 23 x 10-6/°C
The temperature to which cracking becomes visible is dependent on the thickness . Thick films are more prone than thin films to crack. A 5 µm the oxide layer will crack at 320°C whereas a 25 µm thickness will crack at 120°C.
Some times the cracks disappear after a couple of days in room temperature with high humidity. In a dry room cracking occurs at a lower temperature compared to humid rooms.
During the anodizing process higher current density increases the possibility of the crazing defect and higher temperature in the anodizing tank will decrease the probability for crazing.
Colored parts especially the electrolytic colored generally tends to crack less than clear anodizing.
Hot sealing will increase the temperature where cracks happen. To decrease the possibility for crazing while cold sealing a final short hot rinse will decreases the crazing.
If you find this article useful and you would like to know more please contact me blog@aluconsult.com
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Still spots available at the Anodizing Workshop in San Diego
For more information go to Anodizing Workshop
If you find this article useful and you would like to know more please contact me blog@aluconsult.com
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Microstructure of aluminum alloys
The microstructure takes form when a metal goes from its melted formed to a solid metal. This solidification forms a polycrystalline metal with different orientations of the grains. These grains will form the microstructure.
The form of this microstructure depends not only upon the heat transfer but also upon the alloy composition. The two major growth types of microstructures are the dendritic and the eutectic. Both types of growth are present in almost every casting because of a better castability of a near-eutectic or eutectic alloy than that of any other compositions.
The solidification occurs in two stages:
The phase diagram shows what happens during solidification when aluminum is alloyed with silicon. The phase diagram tells nothing about the variation in size of the grains in the microstructure, only which phases the microstructure will consists of for a given composition.
The lines in the diagram show, when a phase transformation takes place under equilibrium condition. A phase transformation can for example be liquid to liquid-solid. Solidification starts when the liquidus line is intersected. Here the equilibrium exists between the liquid and the solid + liquid. When the solidus line is intersected all liquid will be solidified and phase transformations can only take place by diffusion.
Three very common microstructures are found in aluminum alloys are:
Single-phase microstructure
When the melt has a content of silicon less than 1.65 wt% the microstructure will start to solidify as primary a-dendrites. Primary due to the fact that these grains are formed in the beginning of the solidification. These dendrites consist of solid aluminum with up to 1.65 wt% silicon in solid solution.
The microstructure will be single-phase, only consisting of these a-dendrites grown to a homogeneous grain structure. The size of these grains can be very different dependent on the undercooling and the solidification time.
As explained the microstructure will be very fine if the undercooling has been big and hereby caused a high nucleation rate. A fine grained microstructure will give a high strength compared with a coarse grained microstructure.
Two-phase microstructure
The microstructure obtained when the alloy is hypoeutectic with a silicon content between 1.65 - 12.6 wt% will be two-phase. Again when the liquidus line is reached, the primary a-dendrites starts to solidify.
When the eutectic temperature is reached the eutectic phase will begin to solidify between the dendrite arms filling this volume out.
The eutectic phase consists of the primary a-aluminum phase and the silicon phase. This will give a two-phase microstructure where the primary a-aluminum crystals are distributed in the eutectic phase as seen in the figure.
The ratio between the two phases will depend on the content of silicon and also the solidification time.
Single-phase microstructure with intermetallic particles
This microstructure is found in heat treatable aluminum alloys, such as 2xxx series alloy.
The first step is heating the alloy to a temperature just below the solidification temperature. This temperature is held until copper is completely in solid solution with aluminum.
By rapidly cooling to a temperature in the range of 180 – 220°C a supersaturated solid solution can be obtained. A supersaturated solid is a solid with a higher content of the alloying element than if the solid was in the equilibrium state.
During the ageing period elements from the supersaturated solution begin to precipitate. These intermetallic particles can be found in the grain boundaries as seen in the figure.
If you find this article useful and you would like to know more please contact me blog@aluconsult.com
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The form of this microstructure depends not only upon the heat transfer but also upon the alloy composition. The two major growth types of microstructures are the dendritic and the eutectic. Both types of growth are present in almost every casting because of a better castability of a near-eutectic or eutectic alloy than that of any other compositions.
The solidification occurs in two stages:
- Nucleation
- Growth of the formed nucleus
These two stages will compete deciding whether the final microstructure becomes fine or coarse. Nucleation is the first step but will, depending on the melting condition and alloying elements, hastily be overtaken by the crystal growth, as explained below.
Nucleation in a melt will occur when a cluster of metal atoms is ordered in a discrete crystal lattice at or under the solidification temperature. The cluster has a certain critical size, rc, above which it is stable. This critical size rc is dependent on the temperature.
When the nucleus is formed the energy will be lower inside the regular crystal lattice than outside in the melt. Then the nucleus will grow at the expense of the melt due to the driving force, which is the free enthalpy. The lower the energy the more stable is the cluster.
Grain growth consists of adding new atom layers to an already existing layer on the nucleus. When the melt consists of liquid and a certain amounts of stable nuclei growth will start. There are two basic morphologies of growth in aluminum alloys as already mentioned: dendritic and eutectic. Generally, a mixture of both will be present when aluminum alloys solidify.
During nucleation heat will be liberated to the surroundings increasing the temperature of the melt. When the temperature in the melt increases the undercooling will decrease. At a temperature just below the solidification temperature growth will take over and nucleation will cease.
The phase diagram shows what happens during solidification when aluminum is alloyed with silicon. The phase diagram tells nothing about the variation in size of the grains in the microstructure, only which phases the microstructure will consists of for a given composition.
The lines in the diagram show, when a phase transformation takes place under equilibrium condition. A phase transformation can for example be liquid to liquid-solid. Solidification starts when the liquidus line is intersected. Here the equilibrium exists between the liquid and the solid + liquid. When the solidus line is intersected all liquid will be solidified and phase transformations can only take place by diffusion.
Three very common microstructures are found in aluminum alloys are:
Single-phase microstructure
When the melt has a content of silicon less than 1.65 wt% the microstructure will start to solidify as primary a-dendrites. Primary due to the fact that these grains are formed in the beginning of the solidification. These dendrites consist of solid aluminum with up to 1.65 wt% silicon in solid solution.
The microstructure will be single-phase, only consisting of these a-dendrites grown to a homogeneous grain structure. The size of these grains can be very different dependent on the undercooling and the solidification time.
As explained the microstructure will be very fine if the undercooling has been big and hereby caused a high nucleation rate. A fine grained microstructure will give a high strength compared with a coarse grained microstructure.
Two-phase microstructure
The microstructure obtained when the alloy is hypoeutectic with a silicon content between 1.65 - 12.6 wt% will be two-phase. Again when the liquidus line is reached, the primary a-dendrites starts to solidify.
When the eutectic temperature is reached the eutectic phase will begin to solidify between the dendrite arms filling this volume out.
The eutectic phase consists of the primary a-aluminum phase and the silicon phase. This will give a two-phase microstructure where the primary a-aluminum crystals are distributed in the eutectic phase as seen in the figure.
The ratio between the two phases will depend on the content of silicon and also the solidification time.
Single-phase microstructure with intermetallic particles
This microstructure is found in heat treatable aluminum alloys, such as 2xxx series alloy.
The first step is heating the alloy to a temperature just below the solidification temperature. This temperature is held until copper is completely in solid solution with aluminum.
By rapidly cooling to a temperature in the range of 180 – 220°C a supersaturated solid solution can be obtained. A supersaturated solid is a solid with a higher content of the alloying element than if the solid was in the equilibrium state.
During the ageing period elements from the supersaturated solution begin to precipitate. These intermetallic particles can be found in the grain boundaries as seen in the figure.
If you find this article useful and you would like to know more please contact me blog@aluconsult.com
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Hard Anodizing Specifications for the Industry
Sometimes it is difficult to find out how to and what to use in an anodizing specification. There are several specifications but MIL-A-8625 F is probably the most common standard used both in the US and Europe for hard anodizing.
This specification does not restrict the process condition, but only which requirements the hard anodic layer has to pass.
Below you will find the four tests which you should always ask your anodizer about.
Coating thickness
If not specified the coating thickness should be no less than 2 mils (50 µm) and the thickness must not vary more than ± 20%. One of the test methods recommended is ASTM B244 measured on each load run through the process. The size of the test samples should have a width of 3 inches (7.63 cm), minimum length of 3 inches (7.63 cm), and a minimum nominal thickness of 0.032 inches (0.081 cm).
Coating weight
The coating weight should be measured according to ASTM B 137 and must not weight less than 4320 mg/ft² for every 1 mil of unsealed coatings. The frequency of this process control test to be conducted is at least once every month and the number of test samples to be tested is three. The size of the test samples should have a width of 3 inches (7.63 cm), minimum length of 3 inches (7.63 cm), and a minimum nominal thickness of 0.032 inches (0.081 cm).
Abrasion resistance
Abrasion resistance should also be tested on three anodized unsealed test samples at least once every month. The test should be performed according to FED-STD-141, Method 6129. The test samples shall be composed of either 2024-T3 or the alloy used most frequently during the month and should be, whenever possible, anodized on a production load. The size of the test samples should have a width of 4 inches (10.16 cm), minimum length of 10 inches (25.4 cm), and a minimum nominal thickness of 0.032 inches (0.081 cm).
The maximum wear index of 3.5 mg/1000 cycles on aluminum copper alloys with a copper content of 2% or higher. The wear index for all other alloys shall not exceed 1.5 mg/1000 cycles.
Corrosion resistance
The most common corrosion test will be described here, the salt spray test ASTM B117-07 in accordance to MIL-A-8625F.
A lot of the hard anodized parts are not sealed to give the highest wear performance. To get the highest corrosion resistance the parts should be sealed either in hot water or dichromate seal.
A test sample with a nominal thickness of 2 mils must not show any corrosion pits except those within 0.06 inches (1.5 mm) of jigging marks or corners after 336 hours of exposure to neutral salt spray. There should not be more than a total of 15 isolated pits, and the size of the pits should not exceed 0.031 inch in diameter on a test group of samples of five, and not more that 5 isolated pits on each.
This test is the most commonly used, but it is important here to mention that the corrosion test should be application based, especially if the application is used in alkaline environment. The evaluation of the corrosion test should always be in accordance to end user specification.
If you find this article useful and you would like to know more please contact me blog@aluconsult.com
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This specification does not restrict the process condition, but only which requirements the hard anodic layer has to pass.
Below you will find the four tests which you should always ask your anodizer about.
Coating thickness
If not specified the coating thickness should be no less than 2 mils (50 µm) and the thickness must not vary more than ± 20%. One of the test methods recommended is ASTM B244 measured on each load run through the process. The size of the test samples should have a width of 3 inches (7.63 cm), minimum length of 3 inches (7.63 cm), and a minimum nominal thickness of 0.032 inches (0.081 cm).
Coating weight
The coating weight should be measured according to ASTM B 137 and must not weight less than 4320 mg/ft² for every 1 mil of unsealed coatings. The frequency of this process control test to be conducted is at least once every month and the number of test samples to be tested is three. The size of the test samples should have a width of 3 inches (7.63 cm), minimum length of 3 inches (7.63 cm), and a minimum nominal thickness of 0.032 inches (0.081 cm).
Abrasion resistance
Abrasion resistance should also be tested on three anodized unsealed test samples at least once every month. The test should be performed according to FED-STD-141, Method 6129. The test samples shall be composed of either 2024-T3 or the alloy used most frequently during the month and should be, whenever possible, anodized on a production load. The size of the test samples should have a width of 4 inches (10.16 cm), minimum length of 10 inches (25.4 cm), and a minimum nominal thickness of 0.032 inches (0.081 cm).
The maximum wear index of 3.5 mg/1000 cycles on aluminum copper alloys with a copper content of 2% or higher. The wear index for all other alloys shall not exceed 1.5 mg/1000 cycles.
Corrosion resistance
The most common corrosion test will be described here, the salt spray test ASTM B117-07 in accordance to MIL-A-8625F.
A lot of the hard anodized parts are not sealed to give the highest wear performance. To get the highest corrosion resistance the parts should be sealed either in hot water or dichromate seal.
A test sample with a nominal thickness of 2 mils must not show any corrosion pits except those within 0.06 inches (1.5 mm) of jigging marks or corners after 336 hours of exposure to neutral salt spray. There should not be more than a total of 15 isolated pits, and the size of the pits should not exceed 0.031 inch in diameter on a test group of samples of five, and not more that 5 isolated pits on each.
This test is the most commonly used, but it is important here to mention that the corrosion test should be application based, especially if the application is used in alkaline environment. The evaluation of the corrosion test should always be in accordance to end user specification.
If you find this article useful and you would like to know more please contact me blog@aluconsult.com
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Anodizing Workshop
This Anodizing Workshop begins with a short investigation on the types of aluminum because knowing your substrate is most importance when aiming for the best results with anodizing. During the two day seminar, students will be taught the anodizing process from pretreatment to post treatment.
The course will cover bath analysis, handling, quality control and various waste treatment options, along with other hot topics for today's professional anodizers. Following this Anodizing Workshop students will have a well rounded understanding of practical anodizing, supported up by an expanded knowledge about the fundamentals of anodizing in sulfuric acid.
The course will cover bath analysis, handling, quality control and various waste treatment options, along with other hot topics for today's professional anodizers. Following this Anodizing Workshop students will have a well rounded understanding of practical anodizing, supported up by an expanded knowledge about the fundamentals of anodizing in sulfuric acid.
More Information or to Register
For more information on Old Town, includingpictures and activities, visit the Hotels Web Site below:
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.3900 Old Town Avenue, San Diego – Old Town, CA 92110Tel: 619-299-7400
The outline for the two days of workshop is as follows:
If you find this article useful and you would like to know more please contact me blog@aluconsult.com
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Aluminum and Anodizing pages
There are a lot of interesting websites on the Internet both on aluminum and anodizing. Here you will find only a few of them. I have categorized the links so it is easier for you to find the one you need for your searching. Please check back later to get the newest I find which would be of your interest.
Anodizing sites:
The Aluminum Anodizers Council is a place to go for general information about Anodizing. They have a web forum where you can ask questions.
The International Hard Anodizing Association website is more specific about Hard Anodizing.
Qualanod - the European Quality label for Anodic Oxide Coatins on Wrought Aluminium for Architectural Purposes.
Patent Storm is a great place to go if you need to find a patent about anodizing or aluminum.
GDA (Gesamtverband der Aluminiumindustrie) has as its central task to establish acceptance and understanding for the material aluminium and products made from it. You will find a lot of useful information, events and news from the German Aluminum Industry.
Finishing.com is a website were you can post your questions about all kind of finishing, incl. Anodizing.
My own website, AluConsult, where you find a lot of anodizing articles.
Aluminum sites:
aluSELECT is a free and very informative database containing technical information on the most widely used aluminium alloys. It obtain information about the mechanical, physical and chemical properties of 35 wrought and 12 casting alloys.
Key to Metals is the world's most comprehensive metals properties database, developed and designed for professional use worldwide. A lot of the information is free but you can sign up for membership.
European Aluminium Association (EAA) represents the aluminium industry in Europe. It aims to secure sustainable growth of the market for aluminium whilst maintaining and improving the image of the industry.
The Aluminum Association works globally to aggressively promote aluminum as the most sustainable and recyclable automotive, packaging and construction material in today’s market.
aluMATTER is a freely-accessible, award-winning website that provides innovative and interactive e-learning resources for aluminium science and technology.
aluminum Denmark is a professional network for the aluminum industry in Denmark.
Institutes and labs working for the anodizing industry:
Qualital - Italian Certification Office and Lab
And then a couple of links which shows how beautiful Aluminum Surface can be when anodized:
The Beautiful Aluminum Surfaces designed by Bess Kristoffersen (go to metaloverflader).
Aluart® - a new world of exciting possibilities where colour, texture, and finish are combined and infused into solid Aluminium. This technology offers the architect/designer an endless choice of colour options to create new and innovative designs in interior and exterior applications.
If you find this article useful and you would like to know more please contact me blog@aluconsult.com
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Anodizing sites:
The Aluminum Anodizers Council is a place to go for general information about Anodizing. They have a web forum where you can ask questions.
The International Hard Anodizing Association website is more specific about Hard Anodizing.
Qualanod - the European Quality label for Anodic Oxide Coatins on Wrought Aluminium for Architectural Purposes.
Patent Storm is a great place to go if you need to find a patent about anodizing or aluminum.
GDA (Gesamtverband der Aluminiumindustrie) has as its central task to establish acceptance and understanding for the material aluminium and products made from it. You will find a lot of useful information, events and news from the German Aluminum Industry.
Finishing.com is a website were you can post your questions about all kind of finishing, incl. Anodizing.
My own website, AluConsult, where you find a lot of anodizing articles.
Aluminum sites:
aluSELECT is a free and very informative database containing technical information on the most widely used aluminium alloys. It obtain information about the mechanical, physical and chemical properties of 35 wrought and 12 casting alloys.
Key to Metals is the world's most comprehensive metals properties database, developed and designed for professional use worldwide. A lot of the information is free but you can sign up for membership.
European Aluminium Association (EAA) represents the aluminium industry in Europe. It aims to secure sustainable growth of the market for aluminium whilst maintaining and improving the image of the industry.
The Aluminum Association works globally to aggressively promote aluminum as the most sustainable and recyclable automotive, packaging and construction material in today’s market.
aluMATTER is a freely-accessible, award-winning website that provides innovative and interactive e-learning resources for aluminium science and technology.
aluminum Denmark is a professional network for the aluminum industry in Denmark.
Institutes and labs working for the anodizing industry:
Qualital - Italian Certification Office and Lab
And then a couple of links which shows how beautiful Aluminum Surface can be when anodized:
The Beautiful Aluminum Surfaces designed by Bess Kristoffersen (go to metaloverflader).
Aluart® - a new world of exciting possibilities where colour, texture, and finish are combined and infused into solid Aluminium. This technology offers the architect/designer an endless choice of colour options to create new and innovative designs in interior and exterior applications.
If you find this article useful and you would like to know more please contact me blog@aluconsult.com
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Streaking defects on extruded, anodized aluminum
The term streaking is used when a consistent band or elongated mark is found on a profile, producing a non-uniform surface appearance.
There are several kinds of streaking that appear on aluminum profiles. AACOA Inc. has a very good and simple description in their glossary on the various streaking phenomenons.
Streaking can be divided into three categories:
Extrusion process streaks are usually due to process features or process parameters. A common process streak is found on the welding seam of billet-to-billet extrusions and is often concentrated in the beginning part of the extrusion. The surface defects often appear as white dots.
Die streaks depend on the strain and strain rate of the profile, the temperature in the deformation zone in the emerging extrusion and on the cooling rate afterwards.
All these streak defects occur because of a difference in the intermetallic phases, resulting in a different microstructure and grain sizes. These defects are often not discovered before etching and anodizing of the material because they are related to how the different alloying elements behave during the anodizing process.
The profile in the picture shows a discoloration which appears lighter than the rest of the profile, called bearing streaks or die design streaks.
These streaks are usually found in areas where the profile has different wall thicknesses, which results in an uneven cooling.
A H-profile is an example where these streaks could be found. The area where the vertical aluminum meets the horizontal will give a different cooling rate, which allows the grains to differ in size and orientation. These streaks are normally narrow and uniform in shape.
As mentioned above these defects occur because of the variation in response during etching and anodizing. The major intermetallic phases in the 6xxx series alloys are Mg2Si and primary Fe-rich phases.
During etching the Fe-rich phase will act as a more noble area than the aluminum matrix, which will stimulate the dissolution of the surrounding aluminum. This dissolution can result in deep pits that will have a major impact on the optical appearance of the surface.
The Mg2Si will act the other way around by being less noble that the aluminum matrix resulting in small pits that makes the surface look matt.
This is purely a problem for the extruder and there will not be much to do for the anodizer. Some would suggest to re-anodize the work to decrease the defect or to use a lower current density.
If you find this article useful and you would like to know more please contact me blog@aluconsult.com
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There are several kinds of streaking that appear on aluminum profiles. AACOA Inc. has a very good and simple description in their glossary on the various streaking phenomenons.
Streaking can be divided into three categories:
- Billet structure streaks
- Extrusion process streaks
- Die streaks
Extrusion process streaks are usually due to process features or process parameters. A common process streak is found on the welding seam of billet-to-billet extrusions and is often concentrated in the beginning part of the extrusion. The surface defects often appear as white dots.
Die streaks depend on the strain and strain rate of the profile, the temperature in the deformation zone in the emerging extrusion and on the cooling rate afterwards.
All these streak defects occur because of a difference in the intermetallic phases, resulting in a different microstructure and grain sizes. These defects are often not discovered before etching and anodizing of the material because they are related to how the different alloying elements behave during the anodizing process.
The profile in the picture shows a discoloration which appears lighter than the rest of the profile, called bearing streaks or die design streaks.
These streaks are usually found in areas where the profile has different wall thicknesses, which results in an uneven cooling.
A H-profile is an example where these streaks could be found. The area where the vertical aluminum meets the horizontal will give a different cooling rate, which allows the grains to differ in size and orientation. These streaks are normally narrow and uniform in shape.
As mentioned above these defects occur because of the variation in response during etching and anodizing. The major intermetallic phases in the 6xxx series alloys are Mg2Si and primary Fe-rich phases.
During etching the Fe-rich phase will act as a more noble area than the aluminum matrix, which will stimulate the dissolution of the surrounding aluminum. This dissolution can result in deep pits that will have a major impact on the optical appearance of the surface.
The Mg2Si will act the other way around by being less noble that the aluminum matrix resulting in small pits that makes the surface look matt.
This is purely a problem for the extruder and there will not be much to do for the anodizer. Some would suggest to re-anodize the work to decrease the defect or to use a lower current density.
If you find this article useful and you would like to know more please contact me blog@aluconsult.com
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