What Is Salt Spray Testing?

Salt spray testing is an invaluable tool for evaluating the corrosion resistance of materials and coatings, helping industries determine how long their products can withstand corrosive environments.

From automotive and aerospace to marine applications, companies rely on salt spray tests to predict material performance in challenging environments, where exposure to salt, fuktighet, and moisture can dramatically impact durability.

I den här artikeln, we’ll explore the main types of salt spray tests, detailing their procedures, drag, and optimal applications to guide you in selecting the most effective method for your needs.

1. What is Salt Spray Testing?

Salt spray testing is a type of accelerated corrosion testing designed to simulate the effects of harsh environmental conditions in a controlled environment.

This involves spraying a fine mist of salt solution on materials or coatings in an enclosed chamber, allowing manufacturers to observe corrosion behavior over a shorter timeframe than would naturally occur.

By examining material performance in salt-rich environments, industries can evaluate product longevity and make informed choices for applications requiring durable, corrosion-resistant components.

2. Basic Types of Salt Spray Tests

With a range of types available, each test caters to specific environmental conditions, offering essential data on how products will withstand corrosion over time.

Neutral Salt Spray (NSS) Test

De Neutral Salt Spray (NSS) Test is the most widely used type of salt spray test, offering a straightforward way to evaluate the corrosion resistance of materials under neutral pH conditions.

The NSS test has been in use since the 1930s and remains a go-to for assessing corrosion performance due to its simplicity and effectiveness across multiple industries.

Förfarande:

• The test begins with a solution of 5% sodium chloride (NaCl), which is atomized into a fine mist within a closed chamber.
• The pH of the solution is carefully maintained between 6.5 och 7.2 to ensure a neutral environment.
• During the test, the chamber temperature is kept at 35°C (95° F) to promote corrosion without extreme temperature fluctuations.
• Standard test durations vary from 24 timme for preliminary evaluations to 1,000 hours or more for high-corrosion-resistance applications.

Drag:

• Provides continuous salt exposure under controlled conditions.
• Replicates neutral pH environments, commonly found in coastal and urban areas.
• Delivers reproducible results, making it ideal for comparing the corrosion resistance of different coatings and metals.

Applicable Products:

• General metal coatings like galvanized steel, anodized aluminum, och rostfritt stål.
• Commonly tested automotive components, such as frames, parentes, and connectors, which are often exposed to salt from road de-icing.
• Construction materials and equipment designed for outdoor use, including fixtures, railings, and external metal panels.

Common Uses and Limitations:

• NSS is effective for assessing the baseline corrosion resistance of materials and coatings, especially for non-oxidizing environments.
• It’s widely applicable, but it does not fully replicate severe or acidic conditions that might be encountered in industrial settings.

Acetic Acid Salt Spray (AASS) Test

De Acetic Acid Salt Spray (AASS) Test adds acetic acid to the neutral salt solution, creating an acidic environment that accelerates corrosion.

This test is essential for materials that may encounter acidic conditions or urban environments with pollution exposure.

Förfarande:

• A solution of 5% sodium chloride is adjusted with acetic acid, lowering the pH to around 3.1 to create an acidic atmosphere.
• The chamber is set to a temperature of 35°C (95° F), similar to NSS but with heightened corrosive conditions.
• Typical testing durations range from 24 till 500 timme, with longer times used to measure more durable coatings.

Drag:

• The acidic conditions increase the test’s aggressiveness, allowing for a faster corrosion evaluation.
• Replicates environments where materials encounter pollutants or mildly acidic rain, which is common in urban or industrial areas.

Applicable Products:

• Decorative or protective coatings, such as electroplated finishes and anodized aluminum.
• Bil and outdoor fixtures, including door handles, trim, and hardware may experience acidic rain exposure.
• Interior fixtures or decorative elements where high shine and minimal corrosion are important.

Common Uses and Limitations:

• AASS is effective for testing coatings that need additional durability in mildly acidic environments, giving it an advantage over NSS in polluted or urban settings.
• It may not be representative of materials intended for exposure to highly alkaline or neutral conditions.

Copper-Accelerated Acetic Acid Salt Spray (CASS) Test

De Copper-Accelerated Acetic Acid Salt Spray (CASS) Test builds on the AASS test by adding copper chloride to the solution, significantly boosting its corrosive power.

This aggressive approach is ideal for high-performance materials requiring robust corrosion protection in challenging environments.

Förfarande:

• A solution containing sodium chloride, acetic acid, and copper chloride is sprayed within the chamber, adjusting the pH to around 3.1.
• The chamber is heated to 50°C (122° F), increasing the rate of corrosion and delivering results more quickly than neutral tests.
• The duration of this test is typically between 24 och 240 timme, suitable for high-resistance materials.

Drag:

• The combination of copper and acetic acid makes CASS one of the most aggressive salt spray tests available.
• Provides valuable insights for materials and coatings exposed to harsh industrial or marine conditions.
• The elevated temperature and acidic solution mimic severe corrosion conditions.

Applicable Products:

• Chrome-plated parts, durable coatings, and high-performance materials in aerospace and marine industries.
• Automotive exterior components that require high corrosion resistance, such as trim, handtag, and mirrors.
• Heavy-duty coatings on industrial machinery, particularly equipment used in salty or humid environments.

Common Uses and Limitations:

• CASS testing is crucial for products that need extreme corrosion resistance, especially in challenging outdoor or marine applications.
• It is generally too aggressive for low-durability materials, which may fail prematurely in this environment.

Modified Prohesion Test (MPT)

De Modified Prohesion Test (MPT) was developed to simulate real-world, outdoor conditions more accurately. It alternates between salt spray and drying cycles, closely resembling natural exposure cycles.

Förfarande:

• The test alternates between a salt spray exposure period and a drying cycle, creating a realistic fluctuation that materials face in outdoor conditions.
• The salt solution typically has a lower concentration than NSS or AASS, ofta 0.05% NaCl, to better simulate rain or mist.
• Each cycle may last 1 hour of spray followed by 1 hour of dry air exposure, with the total number of cycles depending on the test requirements.

Drag:

• Simulates outdoor environmental cycles, making it ideal for products exposed to fluctuating wet and dry conditions.
• Often used as an alternative to continuous spray tests when testing materials for use in variable outdoor environments.

Applicable Products:

• Outdoor signage, metal railings, and metal panels must endure cyclic rain and sun exposure.
• Coatings and finishes for products in non-coastal outdoor settings.
• Industrial and agricultural equipment exposed to natural outdoor conditions.

Common Uses and Limitations:

• MPT provides a more accurate indication of how materials will perform in real-world outdoor environments, especially for cyclic wet-dry exposures.
• Less effective for environments with continuous high-salt exposure, as seen in marine conditions.

Cyclic Salt Spray Test

De Cyclic Salt Spray Test goes beyond traditional salt spray tests by incorporating drying and humidification phases to better simulate natural environmental cycles.

This test is used for materials that face frequent environmental changes, giving more realistic insights into product durability.

Förfarande:

• Alternates between salt spray, drying, and humidification cycles to mimic natural outdoor environments.
• Cycles can vary in length and composition, tailored to specific environments or requirements, such as coastal or industrial exposure.

Drag:

• The alternating cycles replicate real-world outdoor conditions more closely than continuous tests.
• The test is customizable, allowing it to reflect specific environments based on the product’s intended use.

Applicable Products:

• Automotive and aerospace components undergo diverse weather conditions.
• Structural components in buildings that experience regular rain, fuktighet, and temperature changes.
• Marine hardware is exposed to fluctuating conditions with intermittent saltwater exposure.

Common Uses and Limitations:

• Particularly valuable for products that require multi-condition durability and resistance to corrosion.
• The setup and duration can be more complex than standard NSS or AASS, requiring detailed planning.

3. Conversion of test time to specific years

In salt spray testing, it’s common to interpret test hours as approximate equivalents of real-world exposure, but there’s no universal conversion rate due to the variability in real-world conditions (fuktighet, temperature fluctuations, pollutants).

Dock, here are rough guidelines for NSS, AASS, and CASS tests in terms of how they might translate to real-world years in specific environments:

Neutral Salt Spray (NSS) Test

• 24 timme in NSS is approximately 1 year in mild environments (like interior or low-humidity coastal areas).
• 1000 timme of NSS can correspond to 5-10 år of general outdoor exposure but without harsh conditions.
• For highly corrosive environments, like those with consistent saltwater exposure, NSS hours typically underestimate real-time exposure effects.

Acetic Acid Salt Spray (AASS) Test

• 24 timme of AASS is roughly equivalent to 2 år in a mildly acidic environment, like urban areas with moderate pollution.
• 500 timme of AASS might approximate 5-7 år in an industrial or urban environment where acid rain or air pollution is common.
• This test is particularly valuable for testing finishes and coatings that might encounter acidic conditions.

Copper-Accelerated Acetic Acid Salt Spray (CASS) Test

• 24 timme of CASS is approximately 5-10 år in a highly corrosive environment, such as industrial coastal areas.
• 240 timme of CASS could represent over 15-20 år of real-world exposure in urban industrial environments, making it suitable for extreme durability assessments.

Modified Prohesion and Cyclic Salt Spray Tests

• 100 cykler of cyclic testing can approximate 5-10 år in alternating wet and dry environments, commonly encountered in outdoor settings with rain and sunlight.
• This approximation varies, as cyclic tests are particularly effective at simulating natural conditions over long periods.

4. Advanced Salt Spray Testing Methods

Advanced salt spray testing methods are designed to go beyond basic assessments, simulating more complex, real-world environments where materials and coatings face varying conditions.

These tests add cycles, humidity adjustments, and other factors to offer a more accurate representation of how products endure extended exposure, temperature changes, and other environmental variations.

Cyclic Corrosion Testing (CCT)

Cyclic Corrosion Testing (CCT) is a highly sophisticated testing method that combines multiple environmental conditions, including salt spray, fuktighet, drying, and sometimes even temperature fluctuations.

CCT is favored by industries that require materials to withstand multiple weathering conditions over time.

Förfarande:

• CCT involves alternating between different test conditions, typically including salt spray exposure, drying cycles, and high-humidity phases.
• The test chamber’s temperature and humidity levels vary to simulate different stages of corrosion, often designed to mimic natural day-night cycles.
• A typical CCT cycle may involve a few hours of salt spray exposure, followed by drying and humidification phases, lasting from 24 hours to over 1,000 timme depending on the desired intensity of testing.

Drag:

• Reflects realistic environmental exposure by simulating diverse weather conditions and cycles.
• The alternating cycles provide a comprehensive understanding of corrosion patterns, which is especially useful for coatings and materials exposed to changing conditions.

Fördelar:

• CCT delivers a comprehensive assessment that is closer to real-world corrosion than continuous salt spray tests.
• It’s especially effective for evaluating products exposed to natural weather cycles, such as rain-dry periods.

Humidity-Condensation Salt Fog Test

De Humidity-Condensation Salt Fog Test combines the effects of high humidity and salt fog in a controlled chamber to simulate more extreme corrosive environments.

This test is ideal for materials that are exposed to high moisture levels, as well as those that are likely to encounter high-humidity areas with regular salt exposure, such as coastal locations.

Förfarande:

• The test chamber creates a saturated, high-humidity environment by maintaining a relative humidity of 95-100% and temperatures between 40-60° C (104-140° F).
• Salt fog is introduced periodically, combined with the high humidity to accelerate corrosion.
• Durations for this test vary but are generally shorter due to the intense conditions, often lasting between 24 och 250 timme.

Drag:

• Mimics conditions are found in humid, coastal areas, where salt is prevalent, and moisture levels remain high.
• The condensation effect creates an added layer of realism, as materials not only endure salt but also face continuous moisture, a key factor in many coastal and industrial settings.

Fördelar:

• The combined effects of salt and moisture create an accelerated corrosion environment that closely simulates actual conditions in high-humidity or coastal areas.
• The test is particularly valuable for industries that prioritize long-term resistance in intense environments.

Prohesion Test

De Prohesion Test is a specialized method that uses cyclic exposure to both salt spray and air drying, simulating conditions often experienced by outdoor metals and coatings.

Originally developed for industrial coatings, it is now applied to any product requiring weathering resistance in dynamic conditions.

Förfarande:

• In the Prohesion Test, samples are alternately exposed to a salt spray (0.05% NaCl) and a dry phase to mimic fluctuating natural conditions.
• A typical cycle includes 1 hour of salt spray följt av 1 hour of drying vid rumstemperatur.
• The process is repeated for the specified number of cycles, with tests typically lasting 100 till 500 timme based on the intended application and industry standards.

Drag:

• Simulates natural exposure where metals are subject to periods of wet and dry, rather than continuous exposure to salt spray.
• A lower concentration of the salt solution makes it suitable for products that encounter more moderate environmental stresses.

Fördelar:

• Offers a more accurate reflection of natural weathering cycles, especially for coatings that would be exposed to daily rain-dry periods.
• Lower salt concentration and controlled dry phases help ensure the test doesn’t overestimate corrosion.

Choosing the Right Advanced Salt Spray Test

Selecting the appropriate advanced salt spray test is crucial for ensuring accurate corrosion data that align with real-world conditions. Key factors to consider include:

• Environmental Exposure: Choose based on the expected climate—CCT for multi-climate, humidity-condensation for coastal, and Prohesion for moderate outdoor conditions.
• Materialtyp: Consider the composition of metals and coatings, as certain alloys and finishes respond differently to intense humidity or cyclic salt exposure.
• Product Application: Ensure the test aligns with the material’s intended environment and daily usage.

5. Important Considerations for Salt Spray Testing

Salt spray testing, though widely utilized to evaluate corrosion resistance, requires precise preparation, monitoring, and post-test evaluation to yield meaningful and reliable results.

Following these considerations helps maintain the integrity of the testing process and ensures that results are both consistent and applicable to real-world conditions.

Preparation Before the Test

Careful preparation is essential to ensure that salt spray testing accurately represents the durability of materials under specific conditions. Key preparatory steps include:

en. Sample Selection and Preparation

• Representativeness: Choose test samples that accurately represent the material or coating in its typical form.
  This ensures that the results are relevant to the product’s actual application.
• Cleaning Treatment: Clean samples thoroughly to remove contaminants like oils, dust, or residues that could interfere with test results.
  Dock, avoid excessive cleaning that could alter the surface or material properties.
• Identification and Record: Label each sample and record its dimensions, material composition, and other relevant specifications.
  Documentation is crucial for tracking performance and comparing results over time.

b. Test Equipment Inspection and Calibration

• Equipment Function Inspection: Verify that all testing equipment is functioning correctly.
  Check the spray system, chamber, and heating elements to ensure a consistent environment throughout the test.
• Instrument Calibration: Calibrate sensors, thermometers, and salt concentration meters regularly.
  Accurate readings of salt concentration, temperatur, and humidity are essential for reliable test outcomes.

Control During the Test

Maintaining strict control over environmental conditions throughout the test is vital for producing accurate and reproducible results.
Salt spray test chambers must remain consistent to simulate prolonged exposure accurately.

en. Test Condition Settings

• Salt Spray Type and Concentration: Prepare the salt solution according to test specifications. Commonly, the solution is a mixture of 5% sodium chloride and distilled water, but concentrations vary depending on the test type (TILL EXEMPEL., NSS, CCT).
• Temperature and Humidity: Maintain temperatures around 35° C (95° F) for standard tests like NSS; specialized tests may require higher or fluctuating temperatures.
  Humidity levels should also align with test specifications.
• Spray Method and Amount: Adjust the nozzle to ensure a fine mist and set spray cycles based on the specific test procedure.
  A consistent spray rate—usually 1-2 ml/hour for NSS—is essential for even exposure.

b. Sample Placement and Avoidance of Interference

• Placement Method: Position samples at an angle (typiskt 15-30 degrees) to allow for even exposure to the salt spray and prevent the pooling of salt water, which could distort results.
• Avoid Interference Factors: Avoid placing samples too close to chamber walls or each other, which can obstruct airflow and create inconsistent corrosion conditions.
  Spacing samples evenly ensures uniform exposure.

Post-Test Inspection and Evaluation

Once testing concludes, evaluating the results thoroughly is critical for assessing corrosion resistance.
The process typically involves cleaning, drying, and examining the corrosion effects based on standardized metrics.

en. Cleaning and Drying

• Cleaning Method: Rinse the samples gently to remove salt residues without affecting the corrosion that developed during testing.
  Avoid abrasive cleaning, as it may disrupt corrosion patterns.
• Drying Treatment: Allow samples to air dry in a controlled environment. Excessive heat or pressure should be avoided to preserve the integrity of the test results.

b. Corrosion Degree Assessment

• Evaluation Standard Selection: Use industry-standard criteria such as ASTM, Iso, or JIS to classify and interpret corrosion levels.
  Standards like ASTM B117 and ISO 9227 provide guidelines for measuring rust formation, blistering, and pitting.
• Testing Method Application: Choose evaluation methods appropriate to the type of corrosion encountered.
  Till exempel, blisters on coatings can be measured with a magnifying device, while rust spread may require a digital imaging system for precise calculations.

Other Considerations for Reliable Salt Spray Testing

• Miljöpåverkan: Salt spray testing uses chemicals and resources, so disposing of salt solutions responsibly and minimizing waste is important for environmental safety.
• Data Logging: Recording temperature, fuktighet, and spray rate data consistently allows for better analysis and comparison between test cycles, aiding in repeatability.
• Sample Variability: Natural variations in materials or coating thickness can influence results. Testing multiple samples improves data reliability and minimizes discrepancies.

6. Interpreting Salt Spray Test Results

Salt spray test results provide valuable insights into a material’s or coating’s resistance to corrosion, helping engineers and manufacturers understand potential performance under harsh conditions.

Accurate interpretation of these results requires a thorough analysis of corrosion characteristics, comparison against industry standards, and consideration of test limitations.

Common Metrics and Measurements

Understanding specific metrics in salt spray testing is essential to evaluate the durability and longevity of a material. Key measurements include:

• Time to First Rust (TFR): This is the duration until initial rust spots appear on the surface.
  TFR is often used to gauge how quickly a material starts to corrode under accelerated conditions. A longer TFR generally indicates better corrosion resistance.
• Percentage of Corrosion: This metric assesses the proportion of the material’s surface area affected by rust, grop, or other corrosion forms.
  It’s usually measured after a fixed period (TILL EXEMPEL., 100, 500, eller 1000 timme) and provides an overall view of material degradation.
• Depth of Corrosion: For metals, especially in critical applications like aerospace and automotive, corrosion depth is a crucial factor.
  Deeper corrosion can compromise structural integrity, so materials with minimal depth of corrosion are preferable.
• Visual Evaluation and Grading: Visual inspections of the material’s surface are often conducted according to standard grading systems like ASTM D610 (for rating the degree of rusting) or ISO 10289,
  which assesses the protection and aesthetic properties. Grades typically range from no rust to extensive rust coverage.

Correlating Test Duration with Real-world Conditions

Salt spray tests are accelerated simulations, meaning they expose materials to extreme conditions to predict longer-term corrosion behavior.

Dock, interpreting the real-world equivalent of salt spray test hours requires caution due to the variability in actual environmental factors.

• Industry-Specific Correlation: Some industries use general conversions, som 24 hours of salt spray test exposure equating to one year in a mild marine environment.
  Dock, these estimates vary greatly based on humidity, temperatur, pollutant presence, and other conditions in the actual environment.
• Limitations of Direct Conversion: Medan 1000 hours of salt spray exposure can suggest robust corrosion resistance, this doesn’t translate directly to a specific number of years in every real-world setting.
  The test primarily evaluates relative performance rather than providing an exact lifespan prediction.

Evaluation Criteria Based on Standards

Industry standards provide guidelines to ensure consistency in interpreting salt spray test results.

These standards help benchmark performance and offer criteria for passing or failing based on specific requirements:

• ASTM B117: This standard specifies procedures for conducting neutral salt spray (NSS) tests, including exposure time, salt concentration, and temperature.
  Results under ASTM B117 are often measured by time until rust appears or the percentage of surface corrosion.
• Iso 9227: Similar to ASTM B117, this standard covers neutral and acetic acid salt spray tests (NSS and AASS) and provides assessment criteria.
  Iso 9227 defines methods to assess rust percentages, blistering, and coating adhesion.
• ASTM G85: Covering modified salt spray tests like the Prohesion or CASS (Copper-Accelerated Acetic Acid Salt Spray) tests, ASTM G85 focuses on more aggressive environments.
  These tests are commonly used to evaluate materials with high corrosion resistance, such as those used in marine applications.

Interpreting Visual Characteristics of Corrosion

Corrosion comes in many forms, each with implications for material longevity and structural integrity. Key types of corrosion observed in salt spray tests include:

• Pitting Corrosion: Små, deep pits on the surface indicate that the material may have localized weaknesses.
  Pitting often signals vulnerability to aggressive environments and may impact the material’s structural integrity.
• Blistering: Coatings may develop blisters under prolonged salt spray exposure. Blister size and distribution are assessed to determine the effectiveness of protective coatings.
  Frequent blistering typically suggests poor adhesion or a need for improved coating formulations.
• General Surface Rust: The spread of rust on uncoated metals provides a general measure of susceptibility to corrosion.
  Uniform rusting over time can indicate consistent, though limited, motstånd.

Real-World Applications of Salt Spray Test Results

Manufacturers use salt spray test results to make critical decisions about materials, beläggningar, and potential product improvements. Applications of these results include:

• Material Selection and Coating Development: Test data informs choices in materials and coatings, especially for products used in marine, bil-, and construction applications.
  Higher resistance to salt spray correlates with longer product life and reliability.
• Quality Control and Product Certification: Salt spray testing is often a part of quality assurance, validating that products meet required standards before they’re approved for use.
  Consistent test results across production batches ensure uniform quality.
• Improvement in Corrosion-Resistant Coatings: If a coating fails under testing, manufacturers can modify formulations to enhance performance,
  whether by increasing coating thickness, altering chemical composition, or using additional protective layers.

7. Slutsats

Salt spray testing remains essential for ensuring materials meet industry standards in corrosion resistance.
Understanding the various types, from basic NSS to complex CCT, helps industries select the right test based on specific applications and durability requirements.
By using the appropriate salt spray test, companies can enhance product quality, satisfy customer expectations, and reduce long-term costs due to premature material failure.