Non-Chromate, Sodium Hydroxide Stripping of Inorganic Coatings
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| Overview: | Anodize strippers are used
to remove anodized coatings from parts, such as aircraft landing gears.
The strippers are typically used as the first step in a stripping operation
that includes etching. 390-A is a highly alkaline, chromium-free stripper.
It is a dry, solid, sodium hydroxide mixture that is combined with water
to form an anodize stripping solution.
Chromic acid-based stripping solutions are used at many military installations. Consideration of caustic strippers would be appropriate for any facility that is seeking a process to eliminate chromic acid strippers that are used in removing anodized coatings, provided that surface finishing is not a critical issue. The choice of caustic strippers would be dependent on site-specific stripping requirements (etch, roughness, etc.). Based on the results of laboratory testing, the use of 390-A will result in the complete removal of the anodized coating without exceeding a defined etch limit of 0.35 mils. No intergranular attack or irregular etching of the parts occurred when the solution was used as described below. The stripping bath consists of 2 pounds of 390-A per gallon of solution. Regular titrations must be performed in accordance with manufacturer's instructions. The titrations ensure that the correct concentration of 390-A is maintained in the bath. When the stripping rate of the solution slows, additions of 4 ounces of 390-A per gallon of solution are recommended by the manufacturer to restore the stripping rate. The operating temperature of the bath is 120°F. The solution is held at this temperature during operation and allowed to cool during non-production hours. Parts are immersed into the solution for a total of 20 minutes. This maximum immersion time was established based on the allowable etch for parts. After immersion in the stripping solutions, parts are rinsed and then immersed into an etch solution. After etching, the parts are rinsed in a cold rinse and deoxidized in a bright dip. The parts are then rinsed in a series of cold, warm, and hot rinses. |
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| Compliance Benefit: | Chromic acid is subject to
stringent environmental and worker safety regulations. By eliminating the
use of chromium-based strippers, facilities may meet the requirements of
waste reduction under RCRA, 40 CFR 262, Appendix. It also may help
facilities reduce their generator status and lessen the amount of regulations
(i.e., recordkeeping, reporting, inspections, transportation, accumulation
time, emergency prevention and preparedness, emergency response) they are
required to comply with under RCRA, 40 CFR 262. A facility may decrease
the need for meeting NESHAP requirements under 40 CFR 63.340 - 63.347
and for an air permit under 40 CFR 70 and 40 CFR 71.
The Compliance Benefits listed here are provided as a general guideline and should not be strictly interpreted. Actual Compliance Benefits will vary depending on the factors involved, e.g. the amount of workload involved. |
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| Materials Compatibility: | The solution is safe for use in mild steel or stainless steel tanks. Avoid contacting the solution with aluminum, tin, zinc, and alloys containing those metals. |
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| Safety and Health: | The 390-A stripper is a very
caustic material that reacts violently with water, acids, and other substances.
The stripper contains chemicals that are extremely corrosive to skin tissue.
Contact with the body can result in severe burns. When using the 390-A stripper,
operators should wear protective clothing and equipment including chemical
safety goggles and a full face shield. Additionally, appropriate protective
clothing and chemical-resistant gloves and shoes must be worn.
Respiratory protection is not necessary when using the stripping solution under normal conditions, but a National Institute of Occupational Safety and Health (NIOSH)-approved respirator must be used when handling the dry powder and any time that a mist or spray is generated. Use of the stripping solution does not require any additional ventilation equipment other than that used on an existing stripping line. During use, the solution and the surfaces of the tank containing the solution will be hot. Therefore, operators must be cautious when performing work around the system. Consult your local industrial health specialist, your local health and safety personnel, and the appropriate MSDS prior to implementing any of these technologies. |
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| Benefits: |
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| Disadvantages: |
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| Economic Analysis: | The National Defense Center
for Environmental Excellence (NDCEE), which is operated by Concurrent Technologies
Corporation (CTC), was tasked by the Department of Defense to assist in
the transition and operation of a chromium-free, anodize stripping solution.
No detailed economic analysis was conducted; however, the following conclusions
were made.
Capital costs would depend on the availability and construction materials of the tanks currently in place. If a tank is not constructed of mild steel or stainless steel, an appropriate plastic liner would need to be installed in the tank, or the tank would need to be replaced before the solution could be implemented. Labor costs would be minimal. The bath is created and maintained in a similar manner to the other baths found on the stripping lines, and no special procedures requiring additional labor are necessary. A minor utility cost savings will be achieved through the use of the alternative stripper. The 390-A stripper is held at a temperature of 120°F while chromium-based strippers are maintained at a temperature of 190°F. |
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| NSNs: |
There are multiple MSDSs for most NSNs. |
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| Approving Authority: | Approval is controlled locally,
and the technology should be implemented only after engineering approval
has been granted. Major claimant approval is not required. |
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| Points of Contact: |
David Schario Mark Child |
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| Vendors: | Triple S Chemical Products,
Inc. 3464 Union Pacific Ave Los Angles, CA 90023 800/457-4280 This is not meant to be a complete list, as other suppliers of this type of process may exist. |
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| Source(s): | Concurrent Technologies Corporation. Environmental Metal Plating Alternatives. Implementation Plan Task #3 - Anodize Stripping Solutions. July 14, 1997. |
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