Chromic acid anodize
Chromic acid anodising is at times used a bond preparation rather than corrosion prevention in the aircaraft industry. (Particularly in Europe). The formation of the oxide coating if exposed to the atmosphere particularly a warm and wet atmosphere can lead to hydrolysis of the oxide structure. This is detrimental to the bond strength and durability. The presence of water in the oxide layer can lead to the formation of an oxy-hydroxide which is more massive than the oxide leading to a build up of stresses at a crack tip and can promote crack propagation along the adhesive, metal interface. Ultimately leading to bond failure. Adhesion mode of failure.
If the anodise coating has primer applied to the surface it prevents this degradation of the oxide structure and extends the bond strength and in particular its resistance to a warm wet environment. The finger like structure of the oxide also ensures any crack tip is maintained in the adhesive layer rather than migrating to the metal surface. This structure also acts to diperses any forces at a crack tip effectively stop drilling any crack. Cohesive failure mode.
Sulphuric acid anodizing:
Sulphuric acid anodizing also known as natural, clear or silver anodizing is the most common and widely used solution to produce anodizing. The types of sulphuric acid anodizing are further divided in categories depending on field of application and thickness of the coatings. Coatings of moderate thickness 1.8 µm to 25 µm (0.00007″ to 0.001″) are known as Type II in North America, as named by MIL-A-8625, while coatings thicker than 25 µm (0.001″) are known as Type III, hard coat, hard anodizing, or engineered anodizing. Very thin coatings similar to those produced by chromic anodizing are known as Type IIB.
It is less expensive, more environmentally friendly and easier to dye than chromic acid anodize. Although it offers mild abrasion resistance it is more durable than chromic anodize. Like most anodizes corrosion resistance is excellent. The most desirable feature of this form of anodizing would be the excellent results from dyeing yielding deep and rich colors. The natural color of sulfuric acid anodize will vary depending on the alloy. It can be dyed to almost any color or shade. Sulfuric anodizing provides for several desirable qualities such as corrosion resistance, moderate durability and excellent dye ability, Standards for thin (Soft/Standard) sulfuric anodizing are given by MIL-A-8625 Types II and IIB, AMS 2471 (undyed), and AMS 2472 (dyed), BS EN ISO 12373/1 (decorative), BS EN 3987 (Architectural) . Standards for thick sulfuric anodizing are given by MIL-A-8625 Type III, AMS 2469, BS 5599, BS EN 2536 and the obsolete AMS 2468 and DEF STAN 03-26/1.
Often known as natural, clear or silver anodising, this is the commonest type of anodising, and the description covers a wide range of processes at different levels. The process differs from hard anodising in that the electrolyte temperature is higher and the current density employed is lower. The types of sulphuric acid are sub-divided into classes mainly determined by the field of application. All anodising processes are sealed unless the film is used as a primer for paint or adhesives.
Sulfuric acid is the most widely used solution to produce an anodized coating. It is less expensive, more environmentally friendly and easier to dye than chromic acid anodize. The Type II coating as designated by the Mil-A-8625 standard has a moderate thickness 0.00007” to 0.001”. The sulfuric acid anodize coating is also covered in the AMS 2471 and ASTM B580 specifications. The natural color of sulfuric acid anodize will vary depending on the alloy. It can be dyed to almost any color or shade. Sulfuric acid anodize that is not dyed is commonly referred to as clear anodize. After anodize the solution may be sealed in hot water, nickel acetate, sodium dichromate or ROHS compliant solutions.
This form of anodizing yields coatings under 1 mil thick. Although it offers mild abrasion resistance it is more durable than chromic anodize. Like most anodizes corrosion resistance is excellent. The most desirable feature of this form of anodizing would be the excellent results from dyeing yielding deep and rich colors.
Sulfuric anodizeis formed by using an electrolytic solution of sulfuric acid at room temperature and a current density of 15 to 22 Amps per square foot. The process will run for 30 to 60 minutes depending on the alloy used. This will produce a generally clear coating, depending on sealing, a minimum of 8µm thick.
Sulfuric anodizing provides for several desirable qualities such as:
- Corrosion Resistance (336+ hours salt spray resistance )
- Moderate Durability
- Excellent Dyability (yielding deep, rich colors)
Sulfuric anodize coatings are often sealed to enhance corrosion resistance. Hot water seals produce the clearest sulfuric anodize while sodium dichromate yields a yellow-green appearance but is generally a better seal.
Sulfuric acid is the most widely used solution to produce anodized coating. Coatings of moderate thickness 1.8 µm to 25 µm (0.00007″ to 0.001″) are known as Type II in North America, as named by MIL-A-8625, while coatings thicker than 25 µm (0.001″) are known as Type III, hardcoat, hard anodizing, or engineered anodizing. Very thin coatings similar to those produced by chromic anodizing are known as Type IIB. Thick coatings require more process control, and are produced in a refrigerated tank near the freezing point of water with higher voltages than the thinner coatings. Hard anodizing can be made between 25 and 150 µm (0.001″ to 0.006″) thick. Anodizing thickness increases wear resistance, corrosion resistance, ability to retain lubricants and PTFE coatings, and electrical and thermal insulation. Standards for thin (Soft/Standard) sulfuric anodizing are given by MIL-A-8625 Types II and IIB, AMS 2471 (undyed), and AMS 2472 (dyed), BS EN ISO 12373/1 (decorative), BS EN 3987 (Architectural) . Standards for thick sulfuric anodizing are given by MIL-A-8625 Type III, AMS 2469, BS 5599, BS EN 2536 and the obsolete AMS 2468 and DEF STAN 03-26/1.
Boric/Sulfuric Acid Anodizing
Boric/sulfuric acid anodizing (BSAA) and thin film sulfuric acid anodizing (TFSAA) have been identified as two alternatives for chromic acid anodizing (CAA).Both processes are described and compared to the CAA process. The benefits and disadvantages of each process is discussed. Chromic acid anodizing (CAA), commonly recognized as Type I anodizing, has been used extensively to anodize aircraft parts in the United States, Russia and Great Britain. It was initially developed by Bengough and Stuart in 1923[1,2]. The CAA process produces thin, flexible anodic coatings on aluminum materials with excellent corrosion resistance, paint adhesion, and without significant fatigue loss. This process is particularly advantageous in cases where riveted structures, parts with laps, joints or crevice are to be treated because the chromic acid entrapped in these blind areas will protect them from corrosion. Because of these advantages, the CAA process has become a standard pretreatment for structural adhesive bonding or painting of aluminum materials in both military and commercial aircraft industries (MIL-A-8625F, AMS 2470K, ISO 8076, ISO 8077, BAC 5019, BAC 5884, and HP 4-35) over seven decades.
Chemical Film or Chromate Conversion is widely used plating for the protection on Aluminum and is also known by brand names Alodine and Irritdite. Chemical Film is a chemical process that produces a protective chromate conversion film on aluminum and aluminum alloys. The coating can be applied on the aluminum surface, with or with out color, to form a surface corrosion resistant film when it is dried. The darker coatings providing the greatest corrosion protection. If is usually specified either “clear”, meaning with no color, or “gold”. The Classification per Mil-C-5541 is determined by the exposure to time of the aluminum part to the Chemical Film solution.
Qualified under specification MIL-DTL-81706B, Class 1A, Form II, Method A,B,C; and under Class 3, form II, Method C. The clear Iridite 14-2 finish also qualifies under Class 3, Form II, Method C of the same specification. Qualifies under Specification MIL-C-5541E.
Passivation of Stainless Steal
In years gone by, the process specified was to “pickle and passivate” – a two step method – in order to create a surface on stainless steel that would be resistant to corrosion. Today, we differentiate between the two. It is two separate processes. Pickling (or Chemical Descaling) is done to remove scale. Passivation of stainless steel is done to make the surface more passive and corrosion resistant. Stainless PassivationStainless Passivation
The passivation of stainless steel is a process performed to make a surface passive, i.e., a surface film is created that causes the surface to lose its chemical reactivity. Stainless steel passivation unipotentializes the stainless steel with the oxygen absorbed by the metal surface, creating a monomolecular oxide film. Passivation can result in the very much-desired low corrosion rate of the metal.
The passivation of stainless steel is performed when free iron, oxide scale, rust, iron particles, metal chips or other nonvolatile deposits might adversely affect the metallurgical or sanitary condition or stability of the surface, the mechanical operation of a part, component or system, or contaminate the process fluid.
Passivation is performed on clean stainless steel, providing the surface has been thoroughly cleaned or descaled. Since the term “passivation” is used to describe distinctly different operations or processes relating to stainless steels, it is necessary to define precisely what is meant by passivation.
The titanium cleaning process is performed to remove any foreign particles or to enhance adhesive bonding for titanium or titanium alloy parts. The efficiency of titanium surfaces can usually be maintained without elaborate cleaning procedures. There is generally no need to clean for corrosion protection as is sometimes required with stainless steel, nor does the thin oxide surface film in any way combine with cooling water to form heavy mineral deposits as sometimes occur on copper based alloys. Marine fouling of heat exchanger surfaces is sometimes controlled by chlorine injection. Titanium surfaces are totally unaffected by such treatments. Titanium surface condenser tubing is also kept clean in this way as well as by continuous cleaning systems utilizing rubber balls or nylon brushes, without deleterious effects.
Processes such as acid cleaning and brush cleaning are done to clean titanium. Acid such as hydrochloric acid, sulphuric acid, phosphoric acid, citric acid, nitric acid etc are performed at different conditions and different concentrations depending upon the desired product. In brush cleaning process, stainless steel or titanium wire brushes and pipe are preferred. Careful utilization of titanium’s unique properties will provide many years of maintenance-free service for fabricated equipment. Misapplication of titanium, the use of improper cleaning procedures and other abuses can lead to failure. On the other hand, careful use of some preventive measures, particularly those concerned with corrosion and galling resistance, can significantly extend the useful life of titanium equipment.
Stainless Steel Cleaning:
Stainless steel is easy to clean. For domestic and architectural equipments washing with soap or a mild detergent and warm water followed by clean water rinse is usually quite adequate. An enhanced appearance will be achieved if the cleaned surface is finally wiped dry. Special treatments such as Passivation treatments for removal of free iron and other contamination resulting from handling, fabrication, or exposure to contaminated atmospheres, and Pickling treatments for removal of high temperature scale from heat treatment or welding operations are also performed depending upon the application.