02/09/2014
THE PURPOSE, PROPERTIES, AND METHODS FOR APPLYING ANODIC COATINGS TO ALUMINUM SUBSTRATES.
GENERAL:
Anodizing is an electrochemical process for creating an aluminum oxide coating on the surface of aluminum parts for cosmetic or engineering purposes. Aluminum oxide is a naturally occurring coating always found on aluminum that is exposed to oxygen.. This natural oxide coating forms in a very short time and gradually grows in small amounts over large periods of time. Naturally formed aluminum oxide in not cosmetically pleasing, and serves little purpose. The coating is only 0.0005 mils thick, but the chemical process of anodizing can generate aluminum oxide films of a much greater thickness, ranging from .1- 2.0 mils or much higher for certain hard anodize coatings 3.0-4.0 Mils
This thicker oxide coating is more resistant to corrosion and abrasion than either the underlying aluminum substrate or the aluminum with only its natural oxide coating. In addition, most anodized coatings are porous and have the ability to absorb dyes and pigments before they are sealed. This produces an attractive colored oxide coating, integral to the metal itself, and thus less likely to chip or peel than other applied coatings. With proper care and handling anodized aluminum articles can last a lifetime
The formation of the aluminum oxide coating on aluminum involves two competing processes which occur simultaneously. The aluminum parts are made the anode (positive +), in an electrochemical cell using an acidic electrolyte. As current passes through the solution, oxygen is generated at the anode and immediately reacts with the anodically charged aluminum parts dissolving the surface on the aluminum parts and re-depositing the charged oxygen enriched aluminum on to the part in the form of an aluminum oxide coating. The cathodes, made of lead, graphite, or aluminum, help to control the direction the current is flowing.
While this reaction is occurring, the freshly deposited aluminum oxide coating, which forms in vertically rising cells, which look similar to a bee hive honeycomb are soluble in the electrolyte, and are being dissolved by the acid. The process must be controlled so that the aluminum oxide coating is growing at a faster rate than it is being dissolved.
To achieve this goal several parameters need to be controlled simultaneously.
This includes the concentration and temperature of the electrolyte, agitation of the bath, racking the parts in a manner which ensures even distribution of current by accounting for low and high current density areas of the part, while also eliminating the possibility of air pockets. Control of the rate in which current is applied to the parts (known as ramp Rate) is critical, as the first few minutes in the bath will create the top layer, and the additional coating is deposited below that layer.
Proper positioning of the anodes within the working area of the cathodes, to create a thieving effect, to control the directional flow,(known as Throw, or throwing power) of free flowing electrical current. By using ohms law, and the known deposition rate of anodic coatings of 720 Ampere minutes per 1 Sq. Ft. of surface area to produce .001" thick of coating, the 720 rule can be used fairly accurately to determine the amount of time needed to accomplish the desired thickness on a known surface area of aluminum parts.
Failure to control the bath, or the electrical current, can result in negative consequences like “ Wash away” where the dissolution of the oxide coating is equal to or greater than the rate it is being generated, or “ Burning” where localized heat build up within an anodic cell burns through the cell wall, and has a cascading effect, breaking through cell to cell, and eventually building up at the substrate where it can completely dissolve a small hole, or left undetected large portions of the part. This is not something you want to happen to a customer's prototype part, which already has hundreds of hours of machining into it.
Anodizing requires a freshly cleaned oxide free aluminum surface to obtain satisfying results. Therefore Parts shall be free of all foreign substances, oxides and soils such as greases, oil, paint and welding flux. Parts shall have oxide and other interfering films removed by the use of proper cleaning procedures, Chemical etching and deoxidizing, so the aluminum surface is clean and has a fresh layer of active aluminum. The water break test is the standard test to detect a clean surface.
TYPES OF ANODIZE IN ACCORDANCE WITH MIL-A-8625 F
Chromic Acid Anodizing Type I
Chromic acid anodizing is used as a corrosion resistant coating on structural parts, as a base coating to promote primer adhesion, or as a base coat for structural adhesive bonding. Chromic Acid Anodize has little or no effect on the fatigue strength of finished parts, because the anodic coating is very thin. The typical coating thickness for Chromic Acid Anodize is .00005’-.0002” thick. With the coating being so thin, There is no substantial abrasion resistance.
There are two types of chromic acid anodize specified in MIL-A-8625, Type I coatings are anodized at 40 +/- 2 volts, and Type IB coatings which are anodized at 20 +/- 2 volts. There are 2 classes for each coating type class 1 is un-dyed, and class II is dyed
Chromic Acid Anodize provides substantial corrosion resistance, and because the coating contains Hexavalent Chromium, scratches in the anodic coating will “self heal” that is the chromium in the coating will leech out to the exposed aluminum, providing protection against corrosion.
Chromic Acid Anodize is often used where there is the possibility of solution entrapment in holes, recesses, crevices, or welded seams. The trapped solution will not attack the anodic coating, or cause corrosion, unlike sulfuric acid would, if sulfuric anodize was the process.
Chromic Anodize has an appearance that varies from opaque iridescent to dark gray depending on the type of alloy, and the coating thickness. Chromic Anodize coatings that are dichromate sealed may have a gold or olive green appearance.
Unsealed chromic anodize coatings offer the best adhesion for primers or adhesive bonding. There are time restrictions as to how much time is allowed between anodizing, and the application of primer or adhesive bonding. The Time constraints vary by specification from 48 hours to as little as 8 hrs. Failure to apply primer within the allotted time frame often requires the anodic coating to be stripped and re-anodized, so cautious planning is a must. It is difficult, if not impossible in some cases to have a part chromic acid anodized at one facility, then primed at another facility, and still comply with specification requirements.
The chromic anodize bath is composed of Hexavalent Chromium (Cr+6) a toxic, hazardous substance, and a known carcinogen with strict monitoring and compliance regulations imposed by OSHA and the EPA. Hexavalent chromium is one of the substances banned under the ROHS directive.
The chromic acid concentration is depleted by neutralization with dissolved aluminum. Therefore, free chromic acid and aluminum content must be monitored frequently as the bath ages. Additionally, Hexavalent chromium content (free chromic acid) decreases as the bath ages, while Trivalent Chromium (Cr+3) and aluminum content (Total Chromic acid) increase. As the dissolved aluminum increases, the coating thickness and weight decreases. The bath must be cut and replenished or disposed of before the dissolved alumina reaches a concentration of 10 Grams / Liter
SULFURIC ACID ANODIZING TYPE II
Sulfuric Acid Anodize coatings provide good corrosion resistance, moderate abrasion resistance, and improved adhesion for organic coatings. Sulfuric Anodize coatings may affect the fatigue strength of aluminum, so it should not be used for structural parts, where fatigue strength is critical.
Sulfuric anodize coatings are generally colorless and transparent, allowing them to be dyed to a wide array of colors Alloys containing high manganese and silicon levels tend to give grayish or brownish tint to the coatings. The transparency of the coating decreases with increasing coating thickness. Dyed Sulfuric Anodic coatings should be sealed to prevent staining of the colored coating, and also to increase corrosion resistance and light fastness.
Due to the clarity of the coating, flaws in the surface of the substrate will not be “hidden” or “covered” by the anodize coating, in fact the coating may exaggerate some surface imperfections.
So any machining, sanding, or polishing to improve surface appearance, should be completed prior to anodize
Type II Sulfuric Anodize coatings are typically in the .0003”=.0010” thickness range, with class 1 coatings at the lower end of the range, and class II at the higher end of the range.
Sulfuric Acid Anodize should not be used where there is the possibility of solution entrapment in holes, recesses, crevices, or welded seams. The trapped solution will attack the anodic coating, and cause corrosion of the base material. Proper rinsing techniques can minimize the possibility of solution entrapment, but does not eliminate the threat.
Some aluminum alloys anodize at a faster or slower rate than others, so alloys should be identified on the blue print or purchase order. Also certain alloys are processed at higher voltages, and are processed for longer times than other alloys. Anodize loads should consist of similar alloys so the coating is applied at a consistent rate for all parts.
Anodize voltage and run times vary by alloy, and the desired coating class. Anodize shops often use processing tables that list the alloy type, voltage, and suggested run times by coating class, or they may use the more accurate method of running by current density.
TYPE IIB THIN FILM SULFURIC ACID ANODIZE:
Thin Film Sulfuric Acid Anodize is typically used as an alternative to Chromic Acid Anodize.
With many companies eliminating the use of Hexavalent Chromium, Thin film sulfuric is being called out on blue prints more often. Type IIB may not be used as substitute for Type I or IB coatings specified on the blue print or purchase order (particularly if the anodize is being used for structural adhesive bonding. unless it is specifically permitted in the anodize specification that thin film sulfuric or Boric / sulfuric anodize may be used.
Thin film sulfuric anodize coatings range from .0001”-.0002” thick, at this low thickness, the coating offers little abrasion resistance, however due to the thin coating, fatigue strength of aluminum is not affected the way it would be had conventional sulfuric anodize been applied,
TYPE III HARD ANODIZE COATINGS
Hard anodize coatings are used for applications requiring an extremely hard surface (60-70 Rockwell C) second in hardness only to the Diamond. Excellent Abrasion & corrosion resistance, and high dielectric strength (up to 500 V. per .001" of coating thickness) Hard anodize coatings have a much higher operating cost associated with them as the electrolyte must be cooled to approximately 25º-35ºF, although some additives allow for higher operating temperatures up to 60ºF. the Typical thickness of Hard anodize coatings are approximately .002" Thick.
Unlike other anodize coatings, Hard Anodize must be run by current Density as opposed to running by voltage to obtain predictable results. when hard anodizing for engineering purposes, it is critical to know the function of the part, as sealing the coating will have a detrimental effect on it's abrasion resistance. controlling the current is far more critical, and the process must be monitored so that sharp fluctuations in voltage (which indicates burning) can be addressed.
ANODIC COATING SEALING SOLUTIONS
Sealing of anodic coatings increases the corrosion resistance of the coating, by hydrating the anodic cells to the point that they swell, closing the anodic pore. However, sealing anodize coatings significantly decreases the abrasion resistance. Therefore the engineering purpose of the part should be known prior to sealing, particularly when Hard Anodize is applied.
Un-sealed anodize coatings are porous in nature, and therefore are easily stained. UN sealed anodic coatings should not be handled with bare hands, and should be protected by wrapping in tissue paper. Un-sealed coatings readily accept numerous forms of dyeing techniques, making the anodic coating cosmetically appealing.
Dyed Anodic coatings should be sealed to prevent staining of the colored coating, and also to increase corrosion resistance, and light fastness properties. Anodized parts that will receive a paint coating, but within an extended time period after anodize, should be sealed so that contaminants do not get trapped in the anodic coating
Sodium Dichromate Seal: Offers the best corrosion resistance, has self healing properties due to the Hexavalent chromium content, and provides good adhesion for primer. Due to the chromate content, the color of the normally clear sulfuric anodized coating will be changed; Sulfuric Anodize will have a gold appearance similar to that of yellow chromate. When Chromic anodize is sealed in sodium dichromate it will have more of an olive green appearance, the same with hard anodize, only it will be a much darker color with an olive drab appearance.
For this reason sodium dichromate is typically not used for dyed coatings, as it may alter the intended color appearance. Sodium Dichromate contains Hexavalent Chromium, therefore anodize that is sealed in it does not conform to ROHS requirements. Anodized parts are typically sealed for 15-30 minutes at a temperature range of 200º-210ºF. Shorter seal times are used for parts that will be painted, and longer seal times for parts that will not.
Dilute Chromate Seal: offers slightly less corrosion protection, when compared to Sodium Dichromate seal, but provides better adhesion for primer applications. Dilute chromate seal also contains Hexavalent chromium, therefore it is not ROHS compliant, and it will also change the anodize color the way sodium dichromate seal does, only to a lesser degree. Anodized parts are typically sealed for 10-30 minutes at a temperature range of 180º-200ºF. (Depending on specification requirements) Shorter seal times are used for parts that will be painted, and longer seal times for parts that will not.
Nickel or cobalt Acetate Seal provides good corrosion resistance, and Improves Light fastness protection, making it ideal for dyed anodize coatings used in an architectural setting. Nickel acetate seals come in low, mid-temp, and high temp formulations which can reduce energy costs, and minimize the reduction of abrasion resistance associated with high temp seals. Nickel acetate seal does not contain Hexavalent chromium, so when used on sulfuric anodize coatings it may be ROHS compliant. Nickel acetate seal is not recommended for anodize coatings that are to be painted, as it may interfere with primer adhesion. Temperature ranges vary form low temp seals in the 140º-160ºF range to mid temp seals in the 160º-190ºF range, to High Temp seals in the 190º-212ºF range. Seal times vary from 15-45 minutes depending on seal temperature
HOT DI Water Seal: provides the least amount of corrosion protection of all seals. Hot DI Water sealed anodize improves paint adhesion, does not change the color of the anodic coating, and may be ROHS compliant when used exclusively on sulfuric anodized coatings. Water quality and pH must be monitored frequently. Anodized parts are typically sealed for 15-30 minutes at a temperature range of 200º-212ºF.
Trivalent Chromium seals are fairly new, and gaining popularity. a proprietary chemical solution that is composed of trivalent chromium, as opposed to hexavalent chromium so that it is ROHS compliant. Most Trivalent chromium seals are room temperatures seals, so there are no heating costs associated. Trivalent Chromium seals are typically the same chemical composition as Trivalent chromium conversion coatings (MIL-DTL-5541 E Type II) only used at different concentration levels when used as anodic seals.
FEP / PTFE SEAL: (AKA Teflon coated or Teflon impregnated) coatings are typically used on Type III Hard anodize coatings for lubricity and coefficient of friction purposes. First off it should be noted that Teflon is a registered trademark of DuPont, so unless the product used in the process is made by Dupont, it should be referred to as FEP or PTFE. PTFE is resistant to high temperatures, and is capable of passing 500+ hrs salt spray corrosion resistance testing per ASTM B 117. The PTFE coating may leave a milky white appearance on parts, particularly in entrapment areas. Specifications governing Hard Anodize & PTFE coatings include: MIL-A-8625, Mil-A-63576 and AMS 2482
The process of Hard Anodize and Teflon are divided into 2 general application categories. The first being Hard Anodize coatings sealed with PTFE, where the PTFE is applied after anodize, and allowed to air dry or is baked on to the part. The second coating process known as Teflon
Impregnation or Co deposited Hard Anodize & PTFE involve using an electrolyte with the PTFE as an additive in the electrolyte. The second method is not without controversy. As many chemists believe the molecular structure of PTFE is far to large to it within the confines of an anodic cell under any natural or unnatural circumstance. Furthermore there has been no test results proving that the PTFE resides within the a
