ISO 9227 acetic acid salt spray (AASS) testing is a globally recognized accelerated corrosion test used to evaluate the corrosion resistance of metallic materials, metallic coatings, and painted surfaces in an aggressive acidic salt fog environment. This technical guide covers the test principle, standard parameters, sample requirements, testing procedures, evaluation methods, and typical industrial applications of AASS corrosion testing.
Technical Overview
ISO 9227 is the international standard that specifies the apparatus, reagents, and operating procedures for salt spray corrosion tests. The standard defines three test methods: the Neutral Salt Spray (NSS) test, the Acetic Acid Salt Spray (AASS) test, and the Copper-accelerated Acetic Acid Salt Spray (CASS) test. Each method uses a continuous salt fog, but they differ in solution chemistry, pH, and temperature, which determines their severity and typical field of application.
The AASS method exposes specimens to a continuous fog of a 5% sodium chloride solution whose pH has been acidified to 3.1 to 3.3 with glacial acetic acid. Because the acidic environment accelerates electrochemical corrosion reactions, AASS is significantly more aggressive than the NSS test and is widely used as a faster screening test for decorative electroplated coatings, anodized aluminum, conversion coatings, and paint systems. The results allow manufacturers to compare materials, qualify surface treatments, and monitor the consistency of plating and coating production lines.
Standard Test Conditions for AASS
- Test solution: sodium chloride at 50 g/L ± 5 g/L dissolved in distilled or deionized water (conductivity not exceeding 20 µS/m at 25 °C)
- Salt purity: analytical-grade NaCl with copper and nickel impurities each below 0.001% by mass
- Solution pH: 3.1 to 3.3, adjusted with analytical-grade glacial acetic acid and verified in the collected condensate
- Chamber temperature: 35 °C ± 2 °C
- Fog collection rate: 1.0 to 2.0 mL/h (average 1.5 mL/h) per 80 cm² of horizontal collection area
- Atomizing pressure: typically between 70 kPa and 170 kPa, depending on nozzle design
- Cabinet volume: a minimum of 0.4 m³ is recommended to ensure stable, reproducible climate conditions
- Typical test durations: 24, 48, 72, 96, 144, 168, 240, 480, 720, or 1,000 hours, as defined by the relevant product specification
Testing Samples
Metallic Substrates and Machined Parts
Bare and coated engineering materials such as carbon steel, stainless steel, cast iron, copper alloys, aluminum alloys, and zinc die-castings can be tested. Typical samples include fasteners, stamped parts, castings, machined components, and hardware items. Uncoated metals are often evaluated by mass loss and corrosion morphology, while coated parts are assessed for the onset of substrate corrosion.
Electroplated and Metallic Coated Components
This category includes zinc plating, nickel plating, chromium plating, copper/nickel/chromium decorative systems, tin plating, zinc-nickel and zinc-cobalt alloys, hot-dip galvanized steel, and mechanically deposited coatings. AASS is particularly relevant for decorative plating systems, where the acidified fog rapidly reveals porosity, defects, and under-cutting corrosion at coating discontinuities.
Organic Coating and Paint Systems
Painted panels, powder coatings, e-coat primers, and duplex coating systems are frequently tested. Samples are usually scribed with a controlled scratch (typically a cross or straight line) through the coating to the substrate before exposure, in order to evaluate delamination and corrosion creepage from a damaged area in accordance with ISO 4628-8 and ISO 4628-10.
Anodized Aluminum and Conversion Coatings
Sulfuric acid anodized aluminum components, chromate and phosphate conversion coatings, and passivated zinc surfaces are commonly submitted for AASS testing. Anodized coatings are rated for the number and size of corrosion pits using the chart methods of ISO 8993 or the grid methods of ISO 8994.
Prior to testing, specimens must be carefully cleaned according to the material and coating type, with cut edges and non-significant surfaces protected using inert wax or tape when specified. Specimens should not be over-cleaned or abraded in a way that alters the natural surface condition.
Testing Items
During and after AASS exposure, the following characteristics are examined and documented:
- Appearance of corrosion products, distinguishing white rust (zinc, cadmium, or aluminum corrosion) from red rust (steel substrate corrosion)
- Number, size, and distribution of corrosion spots and stains on the significant surfaces
- Blistering, cracking, flaking, loss of adhesion, and chalking of organic coatings
- Corrosion creepage and under-film corrosion measured from an intentional scribe mark
- Corrosion appearance rating according to ISO 10289, expressed as a protection or appearance rating
- Mass loss of uncoated reference or test specimens, reported as grams per square meter
- Post-corrosion functional checks such as adhesion, torque-tension behavior, or electrical contact resistance where required
- Photographic records of specimens immediately after removal, after rinsing, and after drying
Testing Methods
Preparation of the Test Solution
The required mass of sodium chloride is dissolved in purified water to obtain a concentration of 50 g/L ± 5 g/L. The pH is adjusted to 3.1 to 3.3 by adding glacial acetic acid, and the solution is filtered or decanted before atomization to remove any solids. The pH of the collected fog must fall within the same range, confirming that the acidified solution is not altered by the atomization process.
Specimen Preparation and Placement
Specimens are cleaned and, where applicable, scribed, edge-protected, and identified with a method that survives the fog environment. Inside the chamber, specimens are positioned so that their significant surfaces are inclined between 15° and 30° from vertical. Specimens must not contact each other or any metallic chamber components, must not shield one another from the fog, and must not be placed in the direct line of spray from the nozzle. Condensate dripping from upper specimens or fixtures must not fall onto lower specimens.
Test Execution
The chamber is stabilized at 35 °C ± 2 °C before loading, and the salt fog is then applied continuously for the specified duration. The collection rate, collected solution concentration, and pH are verified at least every 24 hours to demonstrate compliance with the standard. Test interruptions must be minimized, and any necessary opening of the chamber must be recorded in the test report.
Post-Test Treatment and Evaluation
At the end of the exposure period, specimens are removed and gently rinsed in running water no warmer than 40 °C to remove residual salt deposits, then dried in accordance with the specification before final examination. Corrosion assessment is performed visually, with optional magnification, and results are reported as corrosion ratings, creepage distances, mass loss values, or pass/fail statements against the acceptance criteria of the relevant product standard. Reference specimens of known behavior are periodically included to verify chamber performance and inter-laboratory reproducibility.
Applications
AASS testing under ISO 9227 is specified across many industries wherever acidic, chloride-laden service environments must be simulated in an accelerated manner:
- Automotive: decorative trim, fasteners, clips, latches, brake and chassis hardware, and painted body components
- Aerospace: plated hardware, conversion-coated aluminum structures, and cadmium-replacement coating systems
- Electronics and electrical engineering: connectors, terminals, PCB surface finishes, and metal enclosures
- Marine and offshore: fittings, shackles, stainless hardware, and corrosion-protected structural components
- Construction and infrastructure: fence and guardrail fittings, door and window hardware, and fastening systems
- Surface treatment industry: process control and qualification of plating lines, passivates, sealers, and anodizing baths
FAQ
What is the difference between NSS, AASS, and CASS tests?
NSS uses a neutral solution (pH 6.5 to 7.2) at 35 °C and serves as a general corrosion resistance test for most metals and coatings. AASS acidifies the same solution to pH 3.1 to 3.3 with acetic acid at 35 °C, making it more aggressive and suitable for testing plating systems and decorative coatings. CASS further adds copper(II) chloride dihydrate at 0.26 g/L and raises the temperature to 50 °C, producing the most severe attack and is mainly used for decorative copper/nickel/chromium plating and anodized aluminum.
How long does an AASS test take?
The duration is defined by the product specification or purchase agreement. Common durations range from 24 hours for thin coatings to 1,000 hours or more for high-performance systems, with 96, 240, and 480 hours being the most frequently specified checkpoints.
Can AASS results predict real service life?
No. Salt spray testing is an accelerated screening and quality control tool. Because the corrosion mechanisms in a continuous salt fog differ from natural weathering, results cannot be directly converted into years of service life, and they are most meaningful for comparing the relative performance of materials and coatings tested under identical conditions.
How is the difference between ISO 9227 AASS and ASTM G85 A1?
Both standards describe an acetic acid salt spray test with essentially the same solution concentration, pH range, collection rate, and 35 °C operating temperature. The main differences lie in documentation, equipment verification details, and reporting requirements, so laboratories should confirm which standard the customer specification invokes before testing.
What are typical acceptance criteria?
Acceptance criteria depend entirely on the product standard. Common examples include no red rust after 96 hours for zinc-plated parts, a corrosion appearance rating of 9 or better per ISO 10289 for decorative plating, or corrosion creepage from a scribe not exceeding a specified distance, such as 2 mm, for organic coating systems.
What sample sizes and quantities are required?
Standard flat panels of approximately 150 mm × 100 mm are widely used for coating development, while actual production parts may be tested as supplied. Whenever possible, at least three identical specimens should be tested to account for variability, and sample dimensions must allow the required 15° to 30° inclination inside the chamber.