July 29, 2025

Salt Spray Testing

Salt spray testing is an accelerated corrosion test used to evaluate the corrosion resistance of metals, protective coatings, plating, and surface finishes. Test specimens are exposed to a controlled salt-containing fog inside a corrosion chamber for a specified period and then evaluated for corrosion or coating deterioration.

Also known as a salt spray test or salt fog test, the method is widely used for fasteners, automotive components, hardware, powder-coated products, electroplated parts, outdoor products, appliances, and other products where corrosion resistance is an important performance requirement.

Salt spray testing provides controlled and repeatable conditions for comparing materials and surface treatments. The applicable test method, exposure duration, and acceptance criteria should be defined by the relevant standard, product specification, engineering drawing, or customer requirement.

What Is a Salt Spray Test?

A salt spray test exposes a specimen to a fine salt-containing mist inside an enclosed test chamber. Temperature, solution composition, pH, fog collection, specimen positioning, and other conditions are controlled according to the selected test method.

The purpose is to create an accelerated corrosive environment that allows the performance of materials and protective finishes to be evaluated within a defined laboratory period.

Salt spray testing is commonly used for:

Evaluating corrosion resistance
Comparing different coating systems
Assessing electroplated finishes
Checking coating process consistency
Evaluating corrosion around damaged coating areas
Supporting supplier qualification
Verifying customer or engineering specifications
Monitoring production quality

A salt spray test does not directly establish how many years a product will last outdoors. Laboratory exposure and natural service environments involve different corrosion mechanisms and environmental variables.

What Does Salt Spray Testing Measure?

Salt spray testing can reveal several forms of corrosion and coating deterioration.

Red Rust

Red rust typically indicates corrosion of an exposed iron or steel substrate. Specifications may define the maximum allowable amount of red rust or require no red rust during a specified exposure period.

White Corrosion

White corrosion products are commonly associated with zinc and zinc-based coatings. Their appearance may be evaluated separately from corrosion of the underlying steel.

Blistering

Moisture and corrosion processes can cause localized swelling or blister formation beneath an organic coating. Blistering can indicate weaknesses in coating adhesion, surface preparation, or coating integrity.

Corrosion Creep

Some coated specimens are intentionally scribed before testing. After exposure, the spread of corrosion from the scribe may be measured according to the applicable evaluation method.

Peeling and Coating Deterioration

Salt spray exposure can reveal coating separation, peeling, cracking, or other visible deterioration that may affect the protective performance of the finish.

The exact characteristics evaluated depend on the material, coating system, test method, and acceptance criteria.

How Does Salt Spray Testing Work?

The detailed procedure depends on the applicable standard, but a typical salt spray test procedure follows several stages.

1. Sample Preparation

Representative specimens are selected and prepared according to the testing requirements.

Before testing, relevant information may include:

Base material
Coating or plating type
Coating thickness
Surface condition
Specimen dimensions
Required test duration
Acceptance criteria

Some specifications require the coating to remain intact, while others require a controlled scribe through the coating before exposure.

2. Specimen Placement

Samples are positioned inside the salt spray chamber according to the applicable standard.

Specimen orientation is controlled because positioning can influence the amount of salt fog reaching the test surface. Samples should also be arranged so that condensate from one specimen does not improperly affect another.

3. Preparation of the Test Solution

A salt solution is prepared according to the selected test method.

The required salt concentration, water quality, pH, and other solution parameters depend on the standard and whether the test uses NSS, AASS, CASS, or another corrosion method.

4. Salt Fog Exposure

The chamber generates a fine mist that creates a controlled corrosive environment around the specimens.

During testing, operating conditions are maintained within the limits specified by the applicable method. Depending on the standard, monitored parameters can include:

Chamber temperature
Solution concentration
pH
Fog collection rate
Exposure duration

5. Exposure for the Specified Duration

Specimens remain in the chamber for the required exposure period.

Common specifications may require 24, 48, 72, 96, 240, 500, 1,000 hours or longer.

The required number of hours must come from the applicable product or coating requirement. Salt spray standards generally describe the test conditions and procedure, while product specifications or customer requirements frequently establish the required exposure period.

6. Evaluation of Test Specimens

At the specified inspection point or after completing the exposure period, specimens are evaluated according to the agreed criteria.

Evaluation may consider:

Red rust
White corrosion
Blistering
Peeling
Surface deterioration
Corrosion around a scribe
Percentage of affected area

The observed results are then compared with the specified acceptance requirements.

Salt Spray Test Conditions

There is no single set of operating conditions that applies to every type of salt spray test.

For example, Neutral Salt Spray (NSS) commonly uses a neutral sodium chloride solution, while AASS and CASS use different solution chemistry and pH conditions to create more aggressive environments for particular applications.

This distinction matters when specifying a test. A requirement stating only 500-hour salt spray test may be incomplete because the testing laboratory may also need to know:

Test standard
Test method
Sample preparation
Exposure duration
Inspection intervals
Acceptance criteria

These details allow the test to be conducted and evaluated consistently.

Salt Spray Testing Standards

Several standards are used for salt spray corrosion testing.

Standard	Scope / Common Use
ASTM B117	Standard practice for operating salt spray (fog) apparatus
ISO 9227	NSS, AASS and CASS testing of metallic materials and corrosion protection
JIS Z 2371	Japanese standard for salt spray testing
IEC 60068-2-11	Salt mist environmental testing for electrotechnical products
GB/T 10125	Chinese standard covering artificial atmosphere corrosion testing

The correct standard depends on the product specification, market, coating system, customer requirement, or engineering documentation.

ASTM B117 Salt Spray Test

ASTM B117 is one of the most widely referenced salt spray testing practices.

It establishes controlled procedures for operating salt spray apparatus, including requirements related to:

Test chamber conditions
Salt solution
Temperature
Fog collection
Specimen positioning
Test continuity

An important point is that ASTM B117 does not provide one universal pass/fail requirement for all products.

A specification might require a zinc-plated component to complete a defined exposure period without exceeding an allowable level of corrosion. Another product may have different acceptance criteria even when both are tested using ASTM B117.

This is why the test standard and the product acceptance requirement should be identified separately.

ISO 9227 Salt Spray Test

ISO 9227 covers artificial atmosphere corrosion tests for metallic materials with or without corrosion protection.

Three commonly referenced methods are:

Neutral Salt Spray (NSS)

NSS uses a neutral salt solution and is widely applied to metallic materials, coatings, plating systems, and corrosion-protection processes.

Acetic Acid Salt Spray (AASS)

AASS uses an acidified salt solution and is used for certain metallic and decorative coating systems.

Copper-Accelerated Acetic Acid Salt Spray (CASS)

CASS adds copper chloride to an acidified salt solution, creating a more aggressive test environment. It is commonly associated with certain decorative coating systems, including copper/nickel/chromium and nickel/chromium coatings.

For detailed requirements covering these methods, see the ISO 9227 Salt Spray Testing Guide.

NSS vs AASS vs CASS

Although all three methods expose specimens to a salt-containing environment, their test conditions and applications differ.

Method	Full Name	General Application
NSS	Neutral Salt Spray	General metallic materials and protective coatings
AASS	Acetic Acid Salt Spray	Certain metallic and decorative coatings
CASS	Copper-Accelerated Acetic Acid Salt Spray	More aggressive evaluation of certain decorative coatings

A 24-hour CASS test therefore cannot be treated as equivalent to a 24-hour NSS test. The exposure chemistry and severity differ.

The method should be selected according to the relevant specification.

How Long Is a Salt Spray Test?

Salt spray tests may run from several hours to hundreds or thousands of hours.

Common specified durations include:

Test Duration	Typical Context
24 hours	Short-duration corrosion evaluation
48 hours	Basic coating or plating requirements
72–96 hours	General coated or plated components
240 hours	Higher corrosion resistance requirements
500 hours	Extended coating performance requirements
1,000+ hours	Long-duration specifications

These values are examples of commonly encountered test periods. They are not universal salt spray acceptance criteria.

For example, two powder-coated components may both undergo 500 hours of salt spray exposure while having different requirements for blistering, rusting, or corrosion creep.

The correct test duration should therefore be determined from:

Product specifications
Engineering drawings
Coating specifications
OEM requirements
Customer requirements
Applicable industry standards

See Salt Spray Test Requirements by Product Type and Industry for further guidance on selecting test durations.

How Are Salt Spray Test Results Evaluated?

A salt spray test result cannot be determined from the exposure duration alone.

For example:

500 hours ASTM B117

describes the test method and exposure period, but it does not fully define the acceptance criteria.

A complete requirement might also specify:

No red rust on defined surfaces
Maximum permitted white corrosion
Maximum blistering
Maximum corrosion creep from a scribe
Maximum percentage of affected surface
Areas that should be excluded from evaluation

The acceptance criteria should therefore be established before the test begins.

Pass or Fail Criteria

A specimen passes when its condition after the specified exposure meets the defined acceptance criteria.

A failure may involve excessive:

Red rust
White corrosion
Blistering
Peeling
Corrosion creep
Coating deterioration

Visual observations, measurements, photographs, or standardized corrosion rating methods may be used depending on the test specification.

What Products Are Salt Spray Tested?

Salt spray testing is applicable to many products containing corrosion-sensitive metals or protective surface treatments.

Fasteners and Hardware

Common examples include:

Screws
Bolts
Nuts
Washers
Hinges
Locks
Brackets
Door hardware

Coatings such as zinc plating, galvanizing, and other protective finishes are frequently evaluated for corrosion resistance.

Powder-Coated Products

Powder coating salt spray testing can be used for:

Outdoor furniture
Metal frames
Equipment housings
Fencing
Appliances
Architectural components
Industrial equipment

Performance can be affected by coating thickness, pretreatment, curing, adhesion, substrate preparation, and coating formulation.

Automotive Components

Automotive applications may include:

Fasteners
Brackets
Clips
Chassis components
Plated parts
Coated metal components

The applicable requirements are often defined by OEM or component-specific specifications.

Electrical and Electronic Products

Salt spray testing may be applied to corrosion-sensitive metallic components such as:

Enclosures
Brackets
Connectors
Metal housings
Mounting components

Environmental test requirements should be selected according to the intended product and application.

Outdoor and Construction Products

Products exposed to moisture or outdoor environments frequently contain protective finishes designed to reduce corrosion.

Examples include:

Outdoor furniture
Fencing
Railings
Construction hardware
Lighting components
Metal fixtures

Marine-Related Components

Salt-rich environments create significant corrosion challenges for metal components. Salt spray testing may therefore form part of a broader corrosion evaluation program for marine-related hardware and components.

Salt Spray Testing for Powder Coatings

Powder-coated products are a common application for corrosion testing because the coating acts as a protective barrier between the metal substrate and the surrounding environment.

Salt spray exposure can reveal problems related to:

Poor pretreatment
Inadequate coating thickness
Incomplete coverage
Contamination
Poor adhesion
Incorrect curing
Damage to the coating

Some coating specifications require the specimen to be scribed before testing so that corrosion spreading beneath the coating can be evaluated.

The coating formulation alone does not determine salt spray performance. Surface preparation, pretreatment, application, curing, and coating integrity can significantly affect the result.

What Causes a Salt Spray Test to Fail?

When a coated product performs poorly during salt spray exposure, the cause may be related to the material, coating, or manufacturing process.

Common causes include:

Poor surface preparation
Inadequate pretreatment
Insufficient coating thickness
Uneven coating coverage
Incorrect curing
Surface contamination

Failures should be investigated in relation to the location and type of corrosion observed. For a detailed explanation of these failure mechanisms and corrective actions, see Why Products Fail Salt Spray Testing.

Limitations of Salt Spray Testing

Salt spray testing provides an accelerated and controlled corrosion environment, but the test does not reproduce every condition encountered during actual product use.

Real-world corrosion can also be influenced by:

Wet and dry cycles
Temperature changes
UV radiation
Mechanical wear
Industrial pollutants
Chemicals
Humidity fluctuations
Material combinations

A salt spray test therefore should not be interpreted as a direct conversion between laboratory hours and years of outdoor service.

For example, 500 hours of salt spray exposure does not mean that a product will last a specific number of years outdoors.

Salt spray testing is most useful when the test method and acceptance criteria are clearly defined for evaluating or comparing a material, coating, process, or product.

Salt spray testing provides a standardized method for evaluating corrosion resistance under controlled laboratory conditions. Reliable testing requires three elements to be defined clearly: the applicable test method, exposure duration, and acceptance criteria.

When a product specification requires independent laboratory evaluation, salt spray testing services can be used to verify corrosion performance according to the specified standard and test conditions.