SWITCHING FROM NSS TO AASS OR CASS SALT SPRAY TESTING
An Auto Technology salt-fog chamber normally used for Neutral Salt Spray (NSS) can also be used for Acetic Acid Salt Spray (AASS). Returning it to NSS requires thorough cleaning and verification that the chamber meets the applicable test requirements.
1. Materials and Industries That Use AASS
ISO 9227 identifies AASS as especially useful for decorative copper–nickel–chromium and nickel–chromium coatings. It also identifies anodic and organic coatings on aluminum as suitable applications. Salt-spray exposures help reveal coating discontinuities, including pores and other defects, and support checks on coating quality. They do not provide a universal conversion from test hours to service life. [1]
ASTM G85 Annex A1 specifies the continuous acetic acid–salt spray exposure. [2] Although AASS developed through automotive corrosion research, its use extends to building products, aluminum finishing, and bathroom hardware.
AASS Applications Beyond Automotive
- Architectural powder coatings. AASS is used to evaluate corrosion resistance of coated aluminum for architectural applications. Powder-coating manufacturer Thermaset lists ISO 9227 AASS testing for its architectural QUALICOAT polyester coatings, with evaluation after 1,000 hours. [9]
- Anodized aluminum. QUALANOD has included AASS in its specifications for evaluating the corrosion resistance of anodized aluminum. Its architectural applications include window frames and facade coverings, where the finish must retain its appearance during prolonged outdoor exposure. [10] [11]
- Bathroom and shower hardware. Bathroom-hardware manufacturer Wakebath reports weekly AASS, NSS, and CASS testing of metal parts, electroplated products, and powder-coated surfaces. These tests help its quality-control team check the corrosion resistance of hardware finishes before shipment. [12]
2. Why AASS Was Developed
AASS developed from efforts to reproduce service corrosion on decorative plated products more effectively than neutral salt spray. ASTM’s historical account credits C. F. Nixon, an electroplating author whose 1945 paper it cites, with adding acetic acid to salt spray. On copper–nickel–chromium plated zinc die castings, the resulting corrosion resembled damage found on comparable automobile parts in Detroit. The procedure became ASTM B287. [3]
J. H. Hooper, author of the 1957 experimental paper The Acetic Acid Salt Spray Test, compared accelerated exposures with outdoor exposure of plated and anodized specimens. His findings included: [4]
- Acetic-acid tests were most useful for the decorative coating systems studied on zinc-base alloy and brass.
- Hot acetic-acid testing showed a relationship between anodic-film thickness and breakdown rate on the high-purity aluminum specimens.
- For the nickel/chromium and copper/nickel/chromium systems studied on steel, Hooper favored a 24-hour sulfur-dioxide test.
- Conventional neutral salt spray showed little correlation with atmospheric exposure in the rate and type of breakdown for the materials investigated.
Hooper’s results supported AASS for the finishes tested, but performance varied by coating and substrate.
3. Cleaning After AASS and Returning to NSS
The following is Auto Technology’s cleaning procedure for returning its chambers from AASS to NSS service. After cleaning, verify chamber operation against the applicable test specification. For ISO 9227 testing, complete the required collected-solution pH and chamber corrosivity checks before resuming NSS exposure. [5]
Drain and Pre-Rinse
Stop the test and prepare the chamber for cleaning. Drain the AASS solution from the solution tank, solution lines, fog-tower bucket, and other accessible collection points.
Pre-rinse with fresh deionized (DI) water to remove remaining test solution. Include areas where liquid can remain trapped. Replacing the solution-tank contents alone will not flush the entire solution-delivery system.
Scrub and Rinse Accessible Surfaces
Using Soft Scrub Gel Cleanser or an equivalent cleaner recommended by Auto Technology, scrub all accessible surfaces exposed to the AASS solution.
Include the chamber interior, inside the fog tower, solution tank, baffles, specimen racks and supports, drain areas, and accessible wetted components. Pay particular attention to areas where solution or deposits can accumulate.
Rinse thoroughly with DI water to remove loosened deposits and cleaning residue.
Replace Solution Filters
Replace the filters in the solution tank and fog-tower bucket, where fitted. Filters can retain contaminated liquid and deposits after the surrounding system has been drained.
Flush Through the Fog-Generation System
Fill the solution tank with fresh DI water and run the chamber with DI water only, producing a water fog. This flushes the solution lines, atomizer, and fog tower.
Drain and replenish the DI water as needed throughout the flushing process.
Collect Fog Daily and Check pH
Perform fog collections each day and check the collected water’s pH to monitor the removal of residual acetic acid from the AASS exposure.
Continue flushing and checking collected-water pH. Complete the final NSS verification with fresh NSS solution as described in Step 8.
Use Sodium Hydroxide if Needed
Sodium hydroxide (NaOH) may be needed in the DI rinse water to help neutralize and flush out residual acetic acid.
After this step, drain the rinse solution and continue flushing with fresh DI water to remove the remaining alkaline solution and neutralization products.
Allow Two to Five Days for Flushing
Depending on how long the chamber ran AASS and how thoroughly it was scrubbed and rinsed, flushing may take approximately two to five days.
Continue daily fog collections and pH checks throughout this period.
Verify NSS Operation Before Resuming Testing
Once flushing is complete, drain the DI water, fill the solution tank with freshly prepared NSS solution, and run the chamber before loading test specimens.
Verify chamber temperature, collected-solution salt concentration and pH, and fog collection rate against the applicable test requirements.
For testing under ISO 9227, also complete the required chamber corrosivity verification using the reference specimens specified in Clause 7. This establishes that the previous exposure has not significantly affected the chamber’s corrosivity. Record the results before returning the chamber to NSS testing. [5]
For ASTM B117 or another specified method, follow that method’s requirements and any additional customer or laboratory qualification requirements.
4. CASS: Development, Applications, and Chamber Cleaning
Although AASS reproduced the automotive corrosion described above, obtaining the desired damage took approximately 200 hours. The need for a faster production-control test prompted further research. [3]
The American Electroplaters’ Society sponsored Project 15, involving automobile manufacturers, plated-parts suppliers, and plating-chemical suppliers. Researchers exposed panels on Detroit taxicabs and analyzed road slush and rainwater. Copper and iron salts proved particularly aggressive. The findings led to Corrodkote and subsequently to adding copper salts to the acidified salt-spray environment, producing Copper-Accelerated Acetic Acid Salt Spray (CASS). [3]
A later research review dates a Detroit vehicle-exposure phase to winter 1953–1954. ASTM B368 credits AES Project 15 with CASS development and records the standard’s original approval in 1961. [6] [7]
Why Bathroom and Plumbing Manufacturers Use CASS
Bathroom and plumbing manufacturers use CASS to evaluate decorative plated finishes. ASTM B368 was developed specifically for nickel–chromium and copper–nickel–chromium coatings and identifies manufacturing control, specification acceptance, and development of plating processes as uses. Its scope includes these coatings on steel, zinc alloys, aluminum alloys, and plastics, as well as anodized aluminum. [7]
Faucets, tub fillers, shower fittings, and bathroom accessories are examples of products whose plated finishes can be evaluated with CASS. Ningbo SWAN, a manufacturer of kitchen and bathroom faucets, shower systems, bathtub fittings, and accessories, offers CASS testing on request. Wakebath also reports CASS testing as part of its bathroom-hardware quality-control program. [13] [12]
For these manufacturers, CASS provides a controlled corrosion exposure for comparing finishes and checking plating quality before products reach the customer. The coating system and product specification determine whether CASS is suitable. [7]
Use of CASS on plated bathroom hardware does not establish its suitability for porcelain-enamel, acrylic, or fiberglass bathtub surfaces. For materials outside its stated scope, ASTM B368 calls for establishing the test’s suitability and its relationship to service experience before specifying it. [7]
Why CASS Creates a Different Cleaning Problem
CASS deliberately introduces copper salts as a corrosion accelerator. Copper remaining afterward becomes an unwanted contaminant when the chamber returns to NSS.
Although the copper chloride used in CASS is water-soluble, draining and rinsing does not establish that all residual copper has been removed. [8] Copper-containing solution and deposits can remain in filters, solution lines, atomizers, reservoirs, fog-tower components, and other wetted areas. Remaining material can subsequently enter the NSS environment and affect its corrosivity.
Neutralizing residual acetic acid changes the acidity; it does not eliminate copper from the system. An acceptable collected-solution pH therefore does not demonstrate that copper contamination has been removed.
For this reason, dedicated CASS equipment is preferable to routinely switching a chamber between CASS and NSS.
What ISO Requires Before Returning to NSS
ISO 9227 permits return to NSS after thorough cleaning, verification of collected-solution pH, and verification of chamber corrosivity using the prescribed reference-specimen procedure. Following CASS, it additionally recommends checking that copper in the collected solution is below 2.5 mg/L, preferably below 0.5 mg/L. [5]
A chamber previously used for CASS should therefore not be returned to NSS merely because its interior looks clean or its collected-solution pH is acceptable.
References
- ISO 9227 — Scope, applications, and limitations .
- ASTM G85 — Standard Practice for Modified Salt Spray (Fog) Testing .
- ASTM, Manual of Industrial Corrosion Standards and Control, 1973 — Automotive-corrosion chapter, pp. 83–84 and references .
- J. H. Hooper, “The Acetic Acid Salt Spray Test,” Transactions of the IMF, 35(1), 1957, pp. 79–90 .
- ISO 9227 — Section 6.6, cabinet reuse, and Clause 7, corrosivity verification .
- Larraitz Ganborena Imaz — Research thesis containing a historical review of Project 15 and the 1953–1954 exposures .
- ASTM B368 — CASS test, coating applications, and manufacturing uses ; standard preview and historical attribution .
- Sigma-Aldrich — Copper(II) chloride dihydrate, water solubility .
- Thermaset — Architectural QUALICOAT powder coatings and ISO 9227 AASS performance .
- QUALANOD — Published specifications, Section 9.5, corrosion resistance of anodized aluminum .
- QUALANOD — Architectural applications, including window frames and facade coverings .
- Wakebath — AASS, NSS, and CASS testing of bathroom hardware .
- Ningbo SWAN Sanitary — Faucet and bathroom-fitting product range and corrosion testing .
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Auto Technology manufactures salt spray and cyclic corrosion chambers and operates an accredited corrosion testing laboratory for customers who prefer to outsource testing.
