Hard Chromic Anodize Anodizing Review

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Mil-A-8625, Chromic Anodize, (Aluminum)

Anodizing is a conversion of the aluminum surface to practically pure aluminum oxide: the anodic coating.  Type  II is of particular interest to the designer wishing for both the virtues of form and function. This anodic coating is significantly more abrasion and corrosion resistant then the untreated metal. The coating thickness is a combination of both penetration and build-up, in approximately a 50-50 ratio. This coating may be subsequently dyed in a variety of colors, imparting a very decorative finish both in a satin and a polished surface result. Color will vary from light gray to dark gray depending on alloy. Not as readily dyed as sulfuric anodize.

Aluminum alloys are anodized to increase corrosion resistance, to increase surface hardness, and to allow dyeing (coloring), improved lubrication, or improved adhesion. The anodic layer is non-conductive.

When exposed to air at room temperature, or any other gas containing oxygen, pure aluminum self-passivates by forming a surface layer of amorphous aluminum oxide 2 to 3 nm thick, which provides very effective protection against corrosion. Aluminum alloys typically form a thicker oxide layer, 5-15 nm thick, but tend to be more susceptible to corrosion. Aluminum alloy parts are anodized to greatly increase the thickness of this layer for corrosion resistance. The corrosion resistance of aluminum alloys is significantly decreased by certain alloying elements or impurities: copper, iron, and silicon, so 2000, 4000, and 6000-series alloys tend to be most susceptible.

Although anodizing produces a very regular and uniform coating, microscopic fissures in the coating can lead to corrosion. Further, the coating is susceptible to chemical dissolution in the presence of high and low pH chemistry, which results in stripping the coating and corrosion of the substrate. To combat this, various techniques have been developed either to reduce the number of fissures or to insert more chemically stable compounds into the oxide, or both. For instance, sulfuric anodized articles are normally sealed, either through hydro-thermal sealing or precipitating sealing, to reduce porosity and interstitial pathways that allow for corrosive ion exchange between the surface and the substrate. Precipitating seals enhance chemical stability but are less effective in eliminating ion exchange pathways. Most recently, new techniques to partially convert the amorphous oxide coating into more stable micro-crystalline compounds have been developed that have shown significant improvement based on shorter bond lengths.

Some aluminum aircraft parts architectural materials, and consumer products are anodized. Anodized aluminum can be found on mp3 players, multi-tools, flashlights, cookware, cameras, sporting goods, window frames, roofs, in electrolytic capacitors, and on many other products both for corrosion resistance and the ability to retain dye. Although anodizing only has moderate wear resistance, the deeper pores can better retain a lubricating film than a smooth surface would.

Anodized coatings have a much lower thermal conductivity and coefficient of linear expansion than aluminum As a result, the coating will crack from thermal stress if exposed to temperatures above 80 °C. The coating can crack, but it will not peel. The melting point of aluminum oxide is 2050 °C, much higher than pure aluminum's 658 °C. (This and the non-conductivity of aluminum oxide can make welding more difficult.) In typical commercial aluminum anodization processes, the aluminum oxide is grown down into the surface and out from the surface by equal amounts. So anodizing will increase the part dimensions on each surface by half the oxide thickness. For example a coating that is (2 μm) thick, will increase the part dimensions by (1 μm) per surface. If the part is anodized on all sides, then all linear dimensions will increase by the oxide thickness. Anodized aluminum surfaces are harder than aluminum but have low to moderate wear resistance, although this can be improved with thickness and sealing.

Anodizing Process:

Preceding the anodization process, wrought alloys are cleaned in either a hot soak cleaner or in a solvent bath and may be etched in sodium hydroxide (normally with added sodium gluconate), ammonium bifluoride or brightened in a mix of acids. Cast alloys are normally best just cleaned due to the presence of intermetallic substances unless they are a high purity alloy such as LM0.

The anodized aluminum layer is grown by passing a direct current through an electrolytic solution, with the aluminum object serving as the anode (the positive electrode). The current releases hydrogen at the cathode (the negative electrode) and oxygen at the surface of the aluminum anode, creating a build-up of aluminum oxide. Alternating current and pulsed current is also possible but rarely used. The voltage required by various solutions may range from 1 to 300 V DC, although most fall in the range of 15 to 21 V. Higher voltages are typically required for thicker coatings formed in sulfuric and organic acid. The anodizing current varies with the area of aluminum being anodized, and typically ranges from 30 to 300 amperes/meter² (2.8 to 28 ampere/ft²).

Aluminum anodizing is usually performed in an acid solution which slowly dissolves the aluminum oxide. The acid action is balanced with the oxidation rate to form a coating with nanopores, 10-150 nm in diameter. These pores are what allow the electrolyte solution and current to reach the aluminum substrate and continue growing the coating to greater thickness beyond what is produced by auto passivation. However, these same pores will later permit air or water to reach the substrate and initiate corrosion if not sealed. They are often filled with colored dyes and/or corrosion inhibitors before sealing. Because the dye is only superficial, the underlying oxide may continue to provide corrosion protection even if minor wear and scratches may break through the dyed layer.

Conditions such as electrolyte concentration, acidity, solution temperature, and current must be controlled to allow the formation of a consistent oxide layer. Harder, thicker films tend to be produced by more dilute solutions at lower temperatures with higher voltages and currents. The film thickness can range from under 0.5 micrometers for bright decorative work up to 150 micrometers for architectural applications.

The anodic coating types and classes are:

Type I Chromic acid anodizing (conventional)
Type IB Chromic acid anodizing (low voltage method)
Type IC Non-chromic acid anodizing (for use as a non-chromate alternative for Type I and IB coatings)
Type II Sulfuric acid anodizing (conventional)
Type IIB Sulfuric acid anodizing (for use as a non-chromate alternative for Type I and IB coatings)
Type III Hard anodize
Class 1 Non-dyed
Class 2 Dyed

AMS 2472C listed as similar to this specification.

Type 1 & Type 1B coatings:

Class 1 200 Milligrams/sq.ft.

Because of thin deposit that will scratch easily. Can be used for inspection of aluminum forgings or castings by noting evidence of chromic acid bleed out from laps, cracks, seams, etc.

Typical thickness specified  0.00001"- 0.0007"

Type I Conventional coating produced from chromic acid bath. Unless otherwise specified, coating will be sealed. Metal salt sealants should not be used on items that will be painted.

Type 1 Low voltage chromic acid anodizing 22V.

Class 1 Non-dyed (Natural, boiling D.I. water sealing).Class 2 Dyed.

Shall not be applied to aluminum alloys with over 5.0% copper, 7.0% silicon, or total alloying constituents over 7.5%. When copper content is less than 4.6% and for all suitable casting alloys be sure aluminum is tempered (such as -T4 or T6)

Mil-A-8625, Chromic Anodize, (Aluminum)

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