Investigation of aluminum surface treatment technologies by analyzing porosity homogeneity and abrasion resistance values.

The abrasion resistance of anodized aluminum is an important property that is often required for material properties and quality control.

Since its publication in 1953, the military standard MIL-A-8625 - Anodic Coatings for Aluminum and Aluminum Alloys has been the primary anodizing specification (mainly used in the United States), with Section 4.5.5 defining how to quantify the abrasion resistance of electrolytically formed hard anodic oxidation coatings. The procedure referred to as the “Taber test” is the accepted test method for testing, process control, comparative ranking or correlation with end-user performance. 

However, in our country these test principles have not yet been developed and tested, and Hungarian-language literature has not yet been produced. The aim of the research is the practical application and testing of the method. 

In the case of components requiring a wear-resistant surface, the MIL specification prescribes a maximum Taber wear index value of 1.5 mg/1000 cycles for type III hard-coated aluminum surfaces or 3.5 mg/1000 cycles for alloys containing >2% Cu. Application examples: pistons, cylinders, hydraulic drives, valves, sliding parts, hinge mechanisms, cams, gears, rotary joints, insulating plates, explosion protection shields, etc.

The test principle is based on the so-called Taber grinder, which produces wear on the surface of the sample by a combination of rolling and rubbing. A flat, approximately 100 mm square test section with a central hole is mounted on a turntable platform that rotates at a fixed speed on a vertical axis. Before each test, two grinding wheels are fitted with a disc. The grinding wheels are lowered onto the surface of the sample with a load exerting a pressure of 1000 grams per wheel. 
The characteristic wear effect is produced by the contact of the test piece against the sliding rotation of the two grinding wheels. As the turntable rotates, the wheels are moved in the opposite direction of the test section around a horizontal axis that is tangential to the axis of the specimen, while a vacuum system removes loose debris during the test. The resulting wear marks form a pattern of intersecting arcs in a circular track, covering an area of ​​approximately 30 cm2. Wear is defined as cumulative mass loss or mass loss per thousand wear cycles (Taber wear index), where the lower the wear index, the better the wear resistance of the material.
The wear resistance of hard anodic coatings can be affected by factors such as test conditions, type of abrasive, pressure between the test piece and the abrasive, alloy composition, coating thickness, and anodizing or sealing conditions, or both.

The following important questions arose during the research, and we present the answers to them:

• Do samples need to be conditioned?

During testing, it was determined that the samples do NOT need to be conditioned, i.e. kept at a fixed temperature for a specified period of time.

• How often should the grinding wheels be reworked?

When performing the Taber test, the grinding wheels may become clogged depending on the adhesive nature of the material being abraded from the sample. This can change the abrasive properties of the wheels and have a significant impact on the test results. To reduce this variation, it is recommended to resurface the wheels before each test. This ensures that the grinding wheels are standard and that any contamination that may have adhered to the wheel surface during previous tests is removed. However, complying with the required new coating adds significant cost to the test and increases the mass loss of the sample. This issue was corrected in the standard in 2019 and updated with the following note: “Inorganic coatings do not require the grinding wheels to be coated after every 500 test cycles.”

• How can the sample affect the test results?

First, to create a uniform wear pattern, the sample surfaces must be flat and parallel. 
Second, if the panel thickness is not sufficient, the anodic coating can cause the panel to warp. In the wrong grip position, uneven wear can occur and the sample must be discarded. The minimum nominal sample thickness specified in the standard is 1.6 mm. 
Thirdly, drilling or punching the center hole may result in the formation of a small burr that must be removed before mounting the sample. 
Fourthly, the samples must be free from all foreign matter, oxides and contaminants such as greases, oils, paints and welding fluids. Remove oxide and other interfering films by appropriate cleaning procedures before anodizing. During all pre-treatment, anodizing and post-treatment, the samples must be handled by their edges to avoid contamination and mechanical damage.

• How can the anodizing process affect the test results?

The properties and characteristics of hard anodic oxidation coatings can be significantly affected by both the alloy and the method of production. Things to watch out for include pretreatment, electrolyte bath temperature, bath agitation, current density, bath chemistry, time in solution, and processing differences (tanks, racks, distance of sample from cathodes, etc.).
Hard anodizing typically results in a dimensional increase of approximately 50% of the coating thickness on each surface. Anodized coatings are often between 8 μm and 150 μm thick; however, for wear applications, the typical thickness is usually between 40 μm and 60 μm. The outer surface of the anodic coating may be softer or less dense, which can cause greater mass loss during the first 1000 wear cycles than during the remaining cycles. “The anodic coating has a well-known microcellular structure of fine pores, its density can vary with thickness, the surface layer is more porous and softer than the inner layer. 
Abrasion resistance is usually measured on an unsealed anodic oxidation coating. While corrosion resistance is often increased by sealing the coating, it has been observed that sealing or painting can reduce wear resistance by more than 50%!

Testing

Weight loss was recorded immediately after the 10,000-cycle abrasion test and after waiting 1 hour. The following data summarizes the results obtained with Method A (box plot is shown for alloy 6061):

Type B test: The purpose of this was to determine whether the outer surface of the anodized sample was softer or less dense. 
For two different alloys (2024 and 7075), the initial 1000-cycle wear mass loss was in each case higher than the average wear index for the remaining wear cycles.
 

Development of surface preparation and anodizing technology, definition of technological window.

Anodizing (anodic oxidation) is a metal surface treatment process in which aluminum surfaces are provided with a hard and wear-resistant, corrosion-resistant, color-retaining and aesthetic layer. The essence of the process is that aluminum is connected as an anode (positive pole) in a dilute acid (such as phosphoric acid, oxalic acid, sulfuric acid or chromic acid), and then water splitting takes place. Oxygen is evolved at the anode, while hydrogen is produced at the cathode. The advantage of the oxide layer thus formed is that it is deposited from the base metal itself, unlike, for example, galvanizing, where another coating metal adheres to the object. On the one hand, the thickness of the oxide layer determines the final color of the object, which can be controlled by the strength of the electric current or the exposure time.
European standards prescribe a standard layer thickness of 5-10-15-20-25 µm. A thicker layer thickness of 30-35 µm can be achieved with the so-called hard anodizing.
The type of surface protection is usually determined by the area of ​​use, the stresses encountered, the function of the workpiece, material properties, and other conditions.
Török Gépipari Kft. has been dealing with aluminum surface treatment and anodizing since 2015. The launch of the new service was preceded by several years of research/development activity, as the technologies to be introduced - such as the fully automated, crane-track production line specifically for the treatment of aluminum (natural, colored and hard anodizing) - were not previously available in our country, not even in the literature. Although there were some plants, they operated manually. However, to achieve optimal and reproducible quality, it is essential to adhere to very strict production and chemical parameters: cycle times in seconds, parameter controls in decimal values ​​(current, voltage, pH, conductivity, bath temperatures, statistical process control of chemicals, etc.). 
 

The company's strict quality-focused approach and commitment to protecting our environment also presented challenges to the design/construction professionals. The wastewater treatment system developed to treat wastewater generated during operation is also unique on a national level. 

The main difficulty with anodizing is that the process is extremely dependent on the raw material and fluctuations in technological parameters. Compared to other similar surface treatment processes, the layer here is formed from the raw material itself, so the layer that forms is influenced by all the alloying elements of the base metal. Therefore, impurities found in the base metal or that enter the material during production have a significant impact on the quality of the final product. The production system must therefore pay particular attention to monitoring and keeping within narrow limits the parameters that are key to the technology.

Our research has shown that an equally important aspect of anodizing is ensuring the proper surface quality, as the oxide layer forms on the surface of the material, making it clearly visible that there is a correlation between the quality of raw and anodized surfaces. To achieve the expected quality, it is therefore essential to properly condition the workpieces, either mechanically or chemically.

Two main groups of these processes can be distinguished:

Mechanical preparation:

• grinding
• trovalization
• glass bead blasting
• quartz sand blasting

Tests were carried out with cross-tests to see how different surface preparation processes affect the quality of anodizing, especially for different material qualities. The investigation also included changes in layer thickness and changes in the visibility of cutting marks (tool paths).

Mechanical processes

All mechanical preparations strongly modify the homogeneity of the surface, as mechanical pretreatments usually cause drastic changes to the surface. Further tests also revealed that quartz sand is particularly unsuitable for any surface preparation work, as SiO2 grains can be incorporated into the surface during grit blasting, thus preventing good quality anodizing. The appropriate blasting material used for surface treatment is glass beads, which can achieve a particularly homogeneous spray pattern and appropriate surface quality.
It is important to note that different surface finishes also influence the layer measurement results, significantly changing the values ​​measured by commonly used eddy current measuring devices.
As a conclusion, it can be drawn that the base material of the measuring device should have the same surface quality as the workpiece to be measured, as this is the only way to measure constant and accurate layer thickness values.

Chemical pretreatments

Chemical pretreatments are generally accepted in anodizing, such as sodium hydroxide baths, acid pickling and the use of hydrogen peroxide bleach.
Sodium hydroxide reacts relatively violently with aluminum (forming the sodium-[tetrahydroxo-aluminate(III)] complex ion). Its properties are exploited in the immersion bath process to homogenize aluminum parts and reduce the visibility of surface defects, scratches, and chip marks. During the tests, certain raw materials reacted to the bath with a strong oxidation reaction, and sodium hydroxide also tends to mill out tolerances (holes) and threads, so this method must be used with due caution in the case of certain raw materials.
During the tests, examining only the oxidation tendency, the following values ​​were recorded:

It can be said that certain raw materials react violently to chemical pre-treatment, so for workpieces made of such material quality, it is necessary to keep the operation within narrow values, or even omit it altogether.
Chemical bleaching is the last operation phase before anodizing. With its application, oxide residues remaining on the raw metal can be successfully dissolved, provided that all pre-treatment steps are adapted to the given raw material.  Complete oxide-freeness is the basis for proper anodizing, as the contaminated surface may remain stained and inhomogeneous after anodizing. The following table summarizes the times required for deoxidation, in the case of different material qualities.
 

The correlations observed after analyzing the data are clear: the complete deoxidation of metals prone to oxidation takes significantly longer even after slight surface oxidation. Furthermore, the complete breakdown of more stubborn contamination leads to a decrease in productivity due to the longer operation times.

Conclusion:

The majority of the raw materials of automotive components require carefully selected operation phases in the surface preparation processes due to their high alloy content in order to achieve the appropriate quality. Structured surface preparation, removal of cutting marks, homogenization of the material all pose challenges for manufacturing and surface treatment plants, but the desired quality can be achieved by choosing the right technology for the raw material.