Corrosion and Salt Spray Testing ISO 9227

In summary: Salt spray corrosion tests simulate the attack of environmental agents on metallic materials in the laboratory. The reference standard is UNI EN ISO 9227, which defines three main methods (NSS, AASS, CASS) with salt solutions at different pH and temperatures. For more realistic simulations, cyclic tests according to ISO 16701 and ISO 11997-1 are used, which alternate dry, wet, and salt spray phases. FDM Environment Makers designs environmental chambers compliant with these standards, customizable for the automotive, aerospace, construction, and electronics industries.

Corrosion Testing: Why It's Essential

Corrosion is among the most well-known and common causes of deterioration of metallic materials and their alloys. It is a natural electrochemical phenomenon that leads to the progressive degradation of a material's physical and mechanical properties, and is impossible to completely prevent .

Metals and their alloys are present in almost every manufacturing sector: from automotive to aerospace, from electronics to construction. For this reason, corrosion tests , capable of accurately simulating the effects of environmental agents on materials before they are released to the market.

The international reference standard for these tests is ISO 9227, which defines standardized procedures for salt spray testing. In this article, we explore the different types of corrosion tests, the methods defined by the standard, and how to perform them correctly.

ISO 9227: The International Standard for Salt Spray Testing

UNI EN ISO 9227 is the international reference standard for salt spray corrosion testing. It is frequently referenced by other industry standards and is considered the starting point for anyone evaluating the corrosion resistance of metallic materials and coatings.

The standard provides precise indications on:

• The characteristics of the test device (the environmental test chamber)
• The characteristics of the chemical solution to be used
• Operating conditions: chamber temperature, nebulization pressure, sample positioning
• The criteria for analyzing the results

Salt spray testing is performed to detect discontinuities, pores, defects, and damage in organic and inorganic coatings and is a critical tool for manufacturing quality control.

The standard distinguishes three main methods, each with precise operating parameters:

NSS Test — Neutral Salt Spray

The neutral salt spray (NSS) test is the most widely used method and is used for metals and alloys, metallic coatings, conversion coatings, anodic oxide coatings, and organic coatings on metallic materials. The solution used is a 5% sodium chloride solution with a neutral pH (6.5-7.2), nebulized at a temperature of 35 ± 2°C.

AASS Test — Acetic Acid Salt Spray

The acetic acid salt spray test (AASS) is used to evaluate the corrosion resistance of decorative coatings, particularly those used for copper + nickel + chromium, nickel + chromium, or anodic coatings on aluminum. The solution is acidified with glacial acetic acid, bringing the pH to 3.1-3.3.

CASS Test — Copper-Accelerated Acetic Acid Salt Spray

The CASS (Copper-Accelerated Acetic Acid Salt Spray) test is the most aggressive variant. Copper chloride (CuCl₂) is added to the acidified solution, significantly accelerating the corrosion process. It is particularly suitable for testing copper, nickel, and chromium decorative coatings and anodic coatings on aluminum, where resistance to extreme conditions must be verified.

Cyclic Corrosion Testing vs. Continuous Salt Spray Testing

In addition to the tests defined by ISO 9227, there are cyclic corrosion tests, developed to simulate more faithfully and realistically the effect that environmental agents will have on the material during its life cycle.

In the classic continuous salt spray test, environmental conditions remain stable for the entire duration of the test. In cyclic tests, however, different phases are programmed to alternate:

This cyclic approach more accurately reproduces the real-world climates that materials will face in service, making the results more reliable than continuous testing. For more information on how the climatic chambers used in these tests work, consult our dedicated guide.

Application Areas of Corrosion Testing

Salt spray testing is required whenever a component or coating must provide a predictable service life in an aggressive environment:

• Automotive: body paint, underbody components, fittings, electrical connectors. Recognized by OEM specifications and ISO 16750.
• Aerospace and Defense: Aluminum alloys, fasteners, exposed electronic components. Recognized by MIL-STD-810 Method 509.
• Construction and architecture: windows and doors, metal facades, galvanized or powder-coated structural elements.
• Electronics and household appliances: PCBs, contacts, metal casings of devices exposed to humid environments.
• Marine and offshore: Components exposed to marine atmospheres, where chloride resistance is a critical requirement.

The Reference Standards for Corrosion Tests

Several international standards govern corrosion testing procedures. Here are the main ones:

• ISO 9227 — The reference standard for salt spray testing (NSS, AASS, CASS). It is the most cited and used standard globally.
• ASTM B117 — The oldest salt spray testing standard, issued by ASTM in 1939. It is still in effect and is especially authoritative in the United States.
• ISO 16701 — Defines cyclic corrosion tests in humid atmospheres, with alternating wet and dry phases.
• ISO 11997-1 — Specifies methods for cyclic corrosion tests applied to coatings.
• DIN 50021 — Historic German standard for salt spray testing, largely superseded by ISO 9227 but still cited in some specifications.
• JIS Z 2371 — Japanese equivalent standard, used in Asian production chains.
• MIL-STD-810 Method 509 — Salt Spray Test for Military and Aerospace Equipment.

The FDM Solution for Corrosion Testing

FDM Environment Makers has been designing and manufacturing environmental test chambers since 1949, entirely in Italy. Our 40/70T/C180V series environmental chambers can be equipped with salt spray diffusers to conduct tests compliant with ISO 9227 and cyclic testing standards.

Thanks to the innovative revoFACE, corrosion cycles can be fully automated: programming dry, wet, and salt spray phases with precise and repeatable transitions. This is particularly important for cyclic tests, where the precision of the transition between phases determines the reliability of the results.

For specific needs, FDM also creates customized solutions with volumes, temperature ranges, and configurations specific to your industry.

Discover FDM Environmental Chambers or contact us for technical advice.

Frequently Asked Questions About Salt Spray Corrosion Testing

What is the difference between ISO 9227 and ASTM B117?

Both standards regulate salt spray testing, but ASTM B117 (USA, 1939) describes only the neutral solution method (equivalent to NSS) and is more general. ISO 9227 (international) is more recent and comprehensive: it includes NSS, AASS, and CASS, with detailed parameters for each method. In Europe and for the automotive/aerospace industries, ISO 9227 is preferred; in the United States, ASTM B117 still prevails, often referred to in OEM specifications.

How long does a salt spray test last?

Duration depends on the material and the required specification. Typical tests range from 24 to 1,000 hours for NSS, and from 16 to 240 hours for AASS and CASS. Cyclic tests can extend over several weeks with repeated 24-hour cycles. The actual duration is defined by the industry standard or the customer's specifications.

Which test to choose: NSS, AASS or CASS?

The choice depends on the type of coating being tested. NSS is the generic test for metals, alloys, and most organic and inorganic coatings. AASS is used for galvanic decorative coatings (nickel-chromium, copper-nickel-chromium) and anodic oxides on aluminum. CASS is the most aggressive version, chosen when accelerated testing is required on decorative coatings intended for harsh environments.

Do cyclic tests replace continuous salt spray testing?

No, they complement each other. The ISO 9227 continuous test remains the benchmark for production quality control and batch comparison because it is standardized and repeatable. Cyclic tests (ISO 16701, ISO 11997-1) are more realistic and preferred for validating new products or coatings intended for complex environments (automotive, outdoor), where actual corrosion is not a continuous attack but an alternating wet and dry phase.

Authoritative Sources

• ISO/TC 156 – Corrosion of metals and alloys. ISO technical committee responsible for ISO 9227 and the series of standards on metal corrosion.
• ASTM Committee G01 – Corrosion of Metals. ASTM committee that publishes ASTM B117, equivalent and complementary to ISO 9227.
• AMPP – Association for Materials Protection and Performance (formerly NACE International). Global reference association for corrosion science and engineering.
• EFC – European Federation of Corrosion. European Federation of National Corrosion Societies, publishes EFC Working Party Reports.

Frequently Asked Questions about ISO 9227 Corrosion Testing

What is the practical difference between NSS, AASS and CASS?

The three variants of ISO 9227 simulate increasing corrosive environments. NSS (Neutral Salt Spray) uses a 5% NaCl solution at pH 6.5-7.2 at 35°C, a general test for paint coatings, galvanizing, and ferrous materials; typical life 96-1000 hours. AASS (Acetic Acid Salt Spray) acidifies the solution with acetic acid at pH 3.1-3.3, simulating an urban/industrial environment with acid fallout; used for copper-nickel-chromium decorative coatings and tin-plated materials. CASS (Copper-Accelerated Acetic Acid Salt Spray) adds 0.26 g/L of CuCl2_while maintaining a pH of 3.1-3.3 at 50°C, the most aggressive test, acceleration factor ~5x vs. NSS; used for exterior automotive coatings, zinc-aluminum alloys, and anodized aluminum. The real-time vs. test correlation is indicative: 1000 hours NSS ≈ 3-5 years coastal exposure. Product standards (e.g., ASTM B117 equivalent to NSS, QUALICOAT for coil coating) specify durability and acceptance criteria.

How is the audition evaluated at the end of the test?

The evaluation combines several standards. ISO 10289:1999 classifies defects on a scale from 10 (no defects) to 0 (severe corrosion) by evaluating blistering, rust spots, and cracking. ISO 4628-3 quantifies rusting with a Ri0-Ri5 scale (0% → 40-50% area). ISO 4628-2 evaluates blistering (density × size). ISO 2409 cross-cut test verifies post-exposure adhesion: a grid is drawn and adhesive tape is applied; the adhesion is classified 0-5. Undercreep measurements from the scribe line: an X-shaped incision is made with a scalpel down to the substrate. After the test, the corrosion propagation under the film is measured in mm (typical requirement < 2 mm per 1000 h NSS). mass loss per unit surface area after pickling

How repeatable is the salt spray test?

The salt spray test has limited inter-laboratory reproducibility (CoV 20-40% over time until rust), which is why ISO 9227 prescribes rigorous calibration with control specimens made of carbon steel type CR4 or CR-S: they must show a mass loss of 70 ± 20 g/m² after 48 hours NSS and 55 ± 15 g/m² after 24 hours AASS. The spray drop must be 1.5 ± 0.5 mL/h per 80 cm² of collection area, measured with 2 collectors ± 10 cm from the chamber axis. The pH of the collected solution at 25 °C must be within 6.5-7.2 (NSS). Temperature in the exposure zone 35 ± 2 °C. Solution density 1.029-1.036 g/cm³. A well-calibrated chamber guarantees intra-laboratory CoV ≤ 10%. Variability arises from: actual composition of the compressed air (residual oil), deionized water quality (conductivity ≤ 20 µS/cm), specimen position (15-30° from the vertical axis), and door opening during testing.

How do you interpret the cyclic corrosion test compared to continuous NSS?

Cyclic tests (ISO 11997, ISO 14993, SAE J2334) alternate phases of salt spray, humidity, dry-off and sometimes low temperatures, simulating real automotive and outdoor environments more realistically. Example ISO 11997-1 cycle B: 2 h salt spray 35 °C → 2 h dry 60 °C/30% RH → 2 h humidity 50 °C/95% RH, repeated for 40-60 cycles (7 days). Advantages compared to continuous NSS: better correlation with real exposure (factor 3-8x more representative), activation of multiple corrosive mechanisms (pitting, crevice, galvanic), simulation of wet-dry alternation typical of temperate climates. SAE J2334 is a global automotive OEM standard: 1 cycle = 24 h with spray 15 min/day, wet 8 h, dry 15.5 h; Typical lifespan of 80-120 cycles for body-in-white qualification. Modern cyclic corrosion chambers integrate salt spray, humidity, and drying into a single unit, reducing manual transfers and ensuring complete profile traceability.