Corrosion is a persistent challenge to the durability of metals and coatings. As products interact with natural and industrial environments, their surfaces can degrade, impacting performance, safety, and aesthetics. Corrosion test methods, especially the salt spray test, are indispensable in product development, quality assurance, and certification for industries relying on metallic or coated parts. Using standards like ASTM B117 and ISO 9227 ensures reliable, comparable results across the globe. While salt spray testing offers a quick and consistent assessment of corrosion resistance, it is best used in combination with other test methods to achieve a good understanding of material performance in real-life conditions.
So here let’s talk about the key corrosion test methods, focusing on the salt spray (fog) test, including procedures, standards, equipment, costs, and timing, as well as how this widely used method compares to other corrosion resistance tests.
1. What is a Corrosion Resistance Test?
Corrosion tests are procedures developed to assess how a material or product withstands exposure to corrosive environments. Such environments might include natural factors like humidity, salt, acid rain, or industrial pollutants. Corrosion commonly occurs due to electrochemical reactions, such as the oxidation of iron to form rust. These reactions can be accelerated by the presence of water, salts, or acidic compounds.
Corrosion resistance tests are required for evaluating the lifespan and reliability of metals, coatings, and finished products. They help manufacturers and engineers determine if a material can endure the specific environments it will encounter during service. Testing might be performed over extended periods in real conditions or accelerated in controlled chambers to produce results in a shorter time frame.
2. Corrosion Test Methods (Standard, Purpose, Uses, Cost, Time)
There are several established methods for testing the corrosion resistance of coating and metal. Each serves different applications and operates under specific standards, budgets, and timeframes.
1. Salt Spray (Fog) Test
The salt spray (fog) test exposes test samples to a continuous, fine mist of a saline (saltwater) solution inside a controlled chamber. The solution (commonly 5% sodium chloride) is atomized and maintained at a constant temperature and humidity. The mist settles on the surfaces of the samples, simulating a harsh, saline environment such as marine or coastal conditions. Over time, this accelerates the formation of corrosion (rust, pitting, or other degradation), allowing for a rapid assessment of how well a material or coating can resist salt-induced corrosion compared to real-world exposure.
- Purpose: Simulates marine or salty environments to accelerate surface corrosion.
- Standards: ASTM B117, ISO 9227.
- Applications: Widely used for automotive parts, coatings, and fasteners.
- Cost & Time: Moderate cost; test durations from 24 hours up to 1,500 hours or more.
2. Immersion Test
In the immersion test, samples are fully or partially submerged in a specific corrosive liquid, such as saltwater, acids, or alkaline solutions, for a set period. The liquid may remain static or be agitated to simulate flow. This setup mimics environments where materials are in constant or frequent contact with aggressive chemicals, such as in pipelines, storage tanks, or marine equipment. After the exposure period, samples are inspected for mass loss, pitting, surface damage, or other forms of degradation, giving a direct measure of their resistance to the chosen environment.
- Purpose: Assesses corrosion in specific, often harsh, chemical environments.
- Applications: Marine equipment, chemical process plants.
- Cost & Time: Moderate; usually runs from a few days to several weeks.
3. Cyclic Corrosion Test (CCT)
The cyclic corrosion test repeatedly subjects samples to different environmental conditions in a programmed sequence. Common cycles include periods of salt spray (fog), followed by drying (air exposure), and high humidity or condensation phases. By alternating between wet, dry, and humid conditions, the test more closely replicates real outdoor weathering, such as rain, drying sun, dew, and salt exposure, than constant salt spray alone. This method reveals how materials perform under fluctuating real-world conditions where corrosion can accelerate during wet/dry transitions.
- Purpose: Replicates real-world conditions more precisely than constant salt spray.
- Standards: SAE J2334, ISO 16701.
- Applications: Automotive, aerospace.
- Cost & Time: Higher cost; test cycles range from days to weeks.
4. Electrochemical Tests
Electrochemical corrosion tests use electrical measurements to quickly and quantitatively assess corrosion rates and mechanisms. In these tests, a sample acts as an electrode in a cell containing an electrolyte (liquid solution), and a potentiostat applies controlled voltages or currents. Techniques like potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measure how easily the sample corrodes, its corrosion potential, and protective properties of coatings. These tests provide rapid, detailed insights into corrosion behavior and are especially valuable for research and quality control.
- Description: Uses electrical signals to measure corrosion rates.
- Purpose: Provides quantitative data on corrosion mechanisms.
- Applications: Research and rapid screening.
- Cost & Time: Equipment-intensive; data can be obtained in minutes or hours.
5. Atmospheric Exposure Test
For atmospheric exposure tests, samples are placed outdoors in specific real-world environments, such as coastal, industrial, or rural sites, where they are subject to natural weather, pollutant, and climate conditions. Over months or years, the samples experience rain, wind, temperature changes, and pollutants just as they would in actual service. The test provides the most realistic assessment of corrosion resistance, but requires significant time and careful documentation of environmental parameters.
- Purpose: Evaluates corrosion under actual weather.
- Applications: Infrastructure, building materials.
- Cost & Time: Low cost; long duration, often months or years.
6. Humidity Test
The humidity test exposes samples to high relative humidity (often above 95%) at a set elevated temperature inside a sealed chamber. This constant moisture environment accelerates corrosion and is particularly effective for evaluating how materials and coatings withstand prolonged dampness, condensation, and mold or mildew growth. After the test period, samples are examined for blistering, rust, tarnish, and other moisture-induced defects.
- Purpose: Examines corrosion in moist atmospheres.
- Standards: ASTM D2247.
- Applications: Coatings, electronics.
- Cost & Time: Moderate cost; days to weeks.
7. Stress Corrosion Cracking (SCC) Test
The stress corrosion cracking test determines how susceptible a material is to cracking under the combined influence of tensile stress and a corrosive environment. Samples are mechanically loaded (stressed) and exposed to specific chemicals or solutions known to promote SCC, such as chlorides for stainless steels. The test monitors the time and conditions required for cracks to initiate and propagate, helping predict the risk of sudden failure in critical components like pipelines and pressure vessels.
- Purpose: Determines susceptibility to stress-induced cracking.
- Applications: Pipelines, pressure vessels.
8. Intergranular Corrosion Test
The intergranular corrosion test specifically examines how corrosion attacks the grain boundaries of metals, especially stainless steels that may have been improperly heat-treated. Samples are exposed to aggressive chemical solutions (such as nitric acid or copper sulfate mixtures) that preferentially attack grain boundaries. After the test, samples are inspected for loss of material along these boundaries, which can lead to brittle failure even if the metal’s surface appears intact.
- Standard: ASTM A262.
9. Filiform Corrosion Test
The filiform corrosion test assesses the tendency of corrosion to propagate beneath coatings or paints in the form of thread-like filaments. Samples are coated, then intentionally scratched or scored to expose the underlying metal. They are then placed in a controlled environment of high humidity and moderate temperature. If the coating is not sufficiently protective, fine, worm-like corrosion tracks develop from the scratch beneath the coating, indicating underfilm corrosion resistance.
- Purpose: Tests effectiveness of coatings against under-film corrosion.
3. Salt Spray (Fog) Test
Since salt spray fog test is the most widely adopted accelerated corrosion test worldwide, below we are going to introduce it with more details specifically. Manufacturers, laboratories, and quality assurance teams rely on it as a rapid, standardized method for comparing corrosion resistance of coatings and materials, particularly for automotive, construction, and metal finishing applications. Due to its frequent use and strict procedural requirements, it is important to understand its principles, equipment, standards, procedures, and limitations in detail. Now let’s find out what the salt spray test involves, how it is conducted, the equipment used, associated costs and timing, and how it compares with other corrosion resistance testing methods.
What is the Salt Spray Test?
The salt spray test (also known as salt fog test or corrosion protection test) is a standardized method that accelerates corrosion by exposing samples to a fine mist of saline solution in a controlled environment. It is especially useful for evaluating the corrosion resistance of coatings, paints, plating, and metallic materials.
Salt Spray Test Chamber & Machine
To create and maintain the controlled environment required for accurate salt spray testing easily, specialized piece of equipment – salt spray test chambers are designed. The design of salt spray test chambers focuses on providing precise environmental control and ease of use. These machines are indispensable tools for laboratories and manufacturers that need to evaluate corrosion resistance quickly and reliably, following standards like ASTM B117 and ISO 9227. Automated features, robust construction, and comprehensive safety systems all contribute to consistent and reproducible test outcomes.
Modern salt spray chambers provide precise environmental control and safety features:
- Construction: Fiber-reinforced polymer (FRP) chambers with glass wool insulation.
- Components: Control panel for temperature, humidity, and air pressure; sample fixtures (panel holders, hanging rods, V-groove trays); fog generation and collection units; solution reservoir with filtration.
- Automation: Touchscreen HMI panels, programmable timers, Ethernet/Wi-Fi connectivity for remote monitoring, auto-refill systems, and data logging/reporting.
- Safety: Alarms for low solution/air pressure, over-temperature, low water levels, and power protection features.
Salt Spray Test Procedure
- Preparation:
- Samples are cleaned and placed in the test chamber at a specified angle (typically 15–20° from vertical).
- The chamber uses a solution – usually 5% NaCl (sodium chloride) in Type 4 water (ultraviolet sterilized, low conductivity and organic content).
- pH of the solution is maintained within standard ranges (e.g., 6.5–7.2 for neutral salt spray).
- Chamber Setup:
- Temperature is set, commonly 35°C ±2°C.
- Relative humidity is kept around 95% ±2%.
- Air pressure is regulated (typically 0.7 to 1.2 kg/cm²).
- Test Execution:
- Fog is generated by atomizing the salt solution with compressed air through a nozzle.
- Samples are exposed for a specific duration (from 24 hours to over 1,500 hours, depending on requirements).
- The test solution is never reused to avoid contamination.
- Validation:
- Fog collection devices ensure the spray rate and uniformity meet test specifications.
- Regular checks and maintenance of pH, solution levels, and air pressure are performed.
- Completion:
- After the set time, samples are removed, washed, and visually inspected for corrosion products (rust, blisters, etc.).
- Results are compared to acceptance criteria defined in the relevant standards.
SST Corrosion Resistance Testing Application
Industries that use Salt Spray Teasting:
- Automotive Industry
- Aerospace Industry
- Marine Industry
- Construction and Infrastructure
- Electronics and Electrical Industry
- Consumer goods manufacturing
- Defense and military
- Railways and transportation
- Energy and power sector
- Paints and coating industry
Salt Spray Test Standards
Salt spray testing (SST) is governed by a range of international, regional, and industry-specific standards. Each standard defines specific requirements for test procedures, solution compositions, durations, and evaluation criteria. Below are the most commonly referenced standards and their procedures:
1. ISO 9227 Salt Spray Test: Corrosion Tests in Artificial Atmospheres
ISO 9227 is the principal international standard for salt spray testing, specifying methods for exposing test samples to a saline mist environment to assess corrosion resistance of coatings and materials.
Test Types & Procedures:
- Neutral Salt Spray (NSS):
- Solution: 5% sodium chloride (NaCl) in deionized water, pH 6.5–7.2.
- Temperature: Chamber maintained at 35°C.
- Sample Position: Mounted at 15°–30° from vertical.
- Duration: Typically 24 to over 1,000 hours, depending on material and coating.
- Inspection: Visual checks for rust, blistering, or coating failure per ISO 4628.
- Acetic Acid Salt Spray (AASS):
- Solution: 5% NaCl + glacial acetic acid, pH 3.1–3.3.
- Temperature: 35°C.
- Duration: Commonly 24–240 hours.
- Use: For decorative coatings like nickel/chrome.
- Copper-Accelerated Acetic Acid Salt Spray (CASS):
- Solution: 5% NaCl, glacial acetic acid, copper(II) chloride, pH 3.1–3.3.
- Temperature: 50°C.
- Duration: 16–48 hours.
- Use: For high-performance coatings (e.g., anodized aluminum, chrome-plated plastics).
2. ASTM B117 Salt Spray Test: Standard Practice for Operating Salt Spray (Fog) Apparatus
ASTM B117 is the most widely used salt spray test in North America, particularly for neutral salt spray.
Test Procedure:
- Solution: 5% NaCl, pH adjusted to 6.5–7.2.
- Temperature: 35°C.
- Sample Placement: 15°–30° from vertical.
- Duration: 24 to 1,000+ hours, depending on the application and specification.
- Evaluation: Visual inspection for corrosion, pitting, or coating degradation.
3. ASTM G85 Salt Spray Test: Modified Salt Spray (Fog) Testing
ASTM G85 covers advanced salt spray test variants, including cyclic and acidified conditions.
Popular Test Types:
- A1 (Acetic Acid): Similar to AASS; solution acidified with acetic acid.
- A3 (Seawater Acidified): Uses synthetic seawater, acidified to pH 2.8–3.0.
- Cyclic Procedures: Alternate between salt spray, dry, and humid phases.
- Duration: Varies by specification, often 48–1,000 hours.
4. JIS Z 2371 Salt Spray Test: Method of Salt Spray Testing (Japan)
Widely used in Japan, JIS Z 2371 closely mirrors ISO 9227 and ASTM B117.
Test Procedure:
- Solution: 5% NaCl, pH 6.5–7.2.
- Temperature: 35°C.
- Duration: 24–1,000+ hours.
- Evaluation: Visual, per defined grading system.
5. DIN 50021 / DIN EN ISO 9227 Salt Spray Test (Germany and EU)
DIN 50021 is the German implementation of ISO 9227; often referenced in European automotive and industrial sectors.
Procedures:
Identical to ISO 9227 regarding solution, temperature, and evaluation.
6. OEM and Military Salt Spray Test Standards
- GMW 14872 (GM): Cyclic corrosion test including salt spray, humidity, and drying for automotive parts.
- Duration: Cycles lasting several weeks (e.g., 10, 20, or 30 cycles).
- MIL-STD-810: U.S. military standard for environmental testing, including salt fog exposure.
- Duration: Typically 48–96 hours (flexible, based on product).
- IEC 60068-2-11: International standard for salt mist tests on electronic components.
Salt Spray Test Time
Test duration varies based on product requirements and standards:
- Short tests: 24, 48, or 96 hours for rapid screening.
- Extended tests: 500, 1,000, or 1,500 hours to simulate long-term exposure (often interpreted as several years of natural exposure).
- Result evaluation: Samples are checked at intervals (e.g., every 100 or 500 hours) for onset and progression of corrosion.
Salt Spray Test Hours Equivalent To Years
Salt spray test hours are often referenced as a measure of real-world corrosion resistance, but it is not a linear or direct conversion to years of outdoor exposure. The correlation depends on:
- Coating type
- Material
- Environment (marine, industrial, rural)
- Test conditions
Typical Reference Estimates:
- 24 hours SST ≈ 1–6 months in a mild outdoor environment
- 120 hours SST ≈ 1–2 years
- 240 hours SST ≈ 2–5 years
- 500 hours SST ≈ 5–10 years
- 1,000 hours SST ≈ 10–20 years
Note: These are very rough estimates. Salt spray testing is an accelerated test under harsh, continuous conditions that do not account for wet/dry cycles, UV, temperature changes, or biological effects present outdoors. Therefore, SST results should be used for comparative evaluation, not as an exact predictor of field life.
Salt Spray Test Cost
Costs depend on several factors:
- Machine: Small benchtop chambers start from a few thousand dollars; industrial systems can be much higher.
- Operation: Includes cost of salt, deionized water, electricity, maintenance, and labor.
- Testing Services: Outsourced tests are typically billed per sample and per test duration (e.g., $100–$500 for basic runs; more for extended durations or special requirements).
Salt Spray Test Acceptance Criteria
Acceptance criteria define what level of corrosion or coating degradation is considered pass or fail after exposure. These criteria are usually set by:
- Customer specifications
- Industry standards
- OEM requirements
Common Acceptance Criteria:
- No visible red rust on the main surface after a specified number of hours (e.g., no red rust after 96 hours for zinc-plated parts).
- Maximum corrosion creep from a scribe or cut edge (e.g., less than 2 mm from the scratch after 240 hours).
- No blistering, flaking, or delamination of the coating.
- White rust only (for zinc coatings) within a certain time frame; red rust is not acceptable before a certain threshold.
- Percentage of affected area: e.g., less than 5% of the surface shows corrosion after 240 hours.
- Evaluation per ISO 4628: Quantitative assessment of blistering, rusting, cracking, or flaking.
Examples:
- Zinc-plated steel (per ASTM B117): No base metal corrosion (red rust) after 72 hours.
- Powder coated steel (per ISO 9227): No blistering or corrosion of substrate after 500 hours.
- Automotive part (per GMW 14872): Maximum 1 mm underfilm corrosion from any scribe after 30 cycles.
4. Salt Spray Test vs Corrosion Resistance Test
The salt spray test is a type of corrosion resistance test, but the term “corrosion resistance test” refers to a broader set of evaluations. While the salt spray test focuses on simulating a highly saline environment to accelerate corrosion, corrosion resistance tests as a group can include immersion, cyclic, electrochemical, and atmospheric exposure tests. Each method is chosen based on the expected environment and material application.
- Salt Spray Test: Simulates aggressive salt-laden atmospheres, primarily for metals and coatings.
- Corrosion Resistance Test (general): May include assessments in acidic, alkaline, humid, or real-world outdoor conditions.
| Aspect | Corrosion Resistance Test | Salt Spray Test |
|---|---|---|
| Definition | A broad category of tests to evaluate how well a material resists corrosion in various environments. | A specific standardized test that exposes samples to a salt fog (salt spray) environment to accelerate corrosion. |
| Scope | Includes many different methods (salt spray, immersion, electrochemical, atmospheric exposure, etc.). | Only one type of corrosion test focusing on salt fog conditions. |
| Test Environment | Can be chemical immersion, cyclic humidity, electrochemical setup, outdoor exposure, etc. | Controlled saline mist (usually 5% NaCl) in a closed chamber. |
| Purpose | To assess corrosion resistance under diverse conditions tailored to specific applications. | To simulate and accelerate corrosion caused by salty environments, mainly marine or coastal. |
| Standards | Various, depending on test method (e.g., ASTM G1, ASTM G31, ASTM G85, ASTM G61). | Common standards include ASTM B117, ISO 9227. |
| Duration | Varies widely depending on test type and goal (hours, days, months, or years). | Usually shorter, from hours to a few weeks, designed for accelerated testing. |
| Results | Can provide qualitative or quantitative data about corrosion mechanisms, rates, and material behavior in different conditions. | Mainly qualitative assessment of corrosion appearance, time to failure, or coating degradation. |
| Application | Used for in-depth corrosion study, material selection, and development. | Often used for quality control and comparative testing of coatings and surface treatments. |
5. Salt Spray Test vs Other Corrosion Resistance Tests
While the salt spray test is valuable for consistent, repeatable, and rapid comparison of materials and coatings, it does not fully represent all real-world conditions. Other test methods may alternate wet/dry cycles (cyclic corrosion), submerge samples in different chemicals (immersion), or expose them to environmental stress (atmospheric, SCC, intergranular tests).
| Test Method | Environment / Setup | Purpose | Typical Standards |
|---|---|---|---|
| Salt Spray Test | Salt fog (5% NaCl) | Accelerate corrosion for marine-like | ASTM B117, ISO 9227 |
| Immersion Test | Immersion in corrosive liquid | Corrosion in specific chemicals | Varies |
| Cyclic Corrosion Test | Cycles of salt spray + drying | Simulate real environment moisture | SAE J2334, ISO 16701 |
| Electrochemical Test | Electrical measurement in solution | Quantitative corrosion rates | ASTM G5, ASTM G59 |
| Atmospheric Exposure | Outdoor exposure | Real-world corrosion | Varies |
| Humidity Test | High humidity, controlled temp | Moist atmosphere corrosion | ASTM D2247 |
| Stress Corrosion Cracking | Tensile stress + corrosive medium | Detect SCC susceptibility | ASTM G36 |
| Intergranular Corrosion | Chemical exposure + microstructure | Grain boundary corrosion | ASTM A262 |
| Filiform Corrosion | Under coating in humidity | Coating degradation | ASTM D7088 |