Environmental exposure analysis is the process of evaluating a material’s response to accelerated corrosion conditions. Typically this is accomplished by exposure to salt fog, humidity, temperature, drying, and or cyclic exposure to all of the above mentioned conditions. ASTM B117 is the de-facto standard evaluating exposure to salt fog conditions, however the automotive OEMs also have engineering specifications and additional evaluation requirements for parts not governed by the ASTM standard. The laboratory employs the use of dedicated exposure cabinets for neutral salt spray,cyclic corrosion, condensing and non-condensing humidity, thermal shock, and other immersion tests.

What are the different types of corrosion?

By nature, anything that comes in contact with our environment is subject to certain corrosion mechanisms. Corrosion is broadly defined as any mechanism that can cause mass loss to a material. Typically the most talked about materials for which corrosion is a widely problematic condition are metallic materials. Corrosion mechanisms for metallic materials are widely classified into several types such as:

• Galvanic corrosion: where differences in oxidation states of two or more materials (unlike materials) in contact with each other can form a galvanic cell.
• Microbiological corrosion: where bacteria simply “eat” away at certain elements in the metals;
• Erosion corrosion, where an abrasive material in a turbulent environment will simply microscopically “chip” away at a metallic material continually forming active sites for additional corrosion mechanisms.
• Concentration gradient corrosion: where the corrosion mechanisms is caused by the need to establish an equilibrium at an interface where a component concentration gradient is large (such as DI water on steel).

The dominant feature of corrosion of materials is that the initiation point is typically at surfaces where the most active metal exists.

Why do we need environmental exposure testing to understand corrosion of products and materials?

Because corrosion is destructive, corrosion resistance is a paramount consideration in engineering for material selection and subsequent surface protection. And as stated above, because the end environment is also a consideration in product/process engineering, the manufacturing world must consider the likelihood of various levels of corrosive environment exposure when designing the products and surface protection. Ultimately, the only way to truly characterize a product’s behavior in a particular environment is to expose to that environment and observe, however that approach is not efficacious when designing to long product lifetimes or are testing variations in the design process. Fortunately we can “simulate” these environments with accelerated versions of the environments, while maintaining standardized conditions as defined by the referenced test methods. These accelerated environments do not provide correlation to the real world environments for which they are meant to simulate, however empirical correlation is often establish by the referenced specifications as well as exposure time durations defined by these empirical determinations.

Types of accelerated corrosion testing:

Typical accelerated corrosion experiments will revolve around four types of cabinets:

• Thermal cabinet: such as a freezer or oven which will only control temperature in a pristine and ambient environment.
• FOG cabinet: where a mist of solution is introduced into the cabinet at a predefined temperature.
• Humidity cabinet: where the temperature and relative humidity can be programmed and controlled;
• Cyclic corrosion cabinet (CCT): where a combination of all exposures can be programmed in combined in cycles (typically a 24 hour cycle).

Ways to establish response to the accelerated corrosion environments are items such as mass loss, appearance of ferrous corrosion (red surface deposition observation), appearance of non-ferrous corrosion (white or black surface deposition observation), functionality of components (actually energizing or mechanizing the assembly after exposure), and adhesion testing, where surface treatments and coatings are evaluated for their bond integrity.

Common standards for accelerated corrosion testing:

In the manufacturing world, accelerated corrosion specifications spelled out are countless, however they tend to cite cabinet operations and conditions defined in industry standards such as ASTM and SAE.
The overwhelming majority of standards are designed around relatively few exposure specifications as detailed below:

• ASTM B117 – Standard Specification for operation of Salt (FOG) cabinet (neutral salt spray)
• ASTM D1735 - Standard Practice for Testing Water Resistance of Coatings Using Water Fog Apparatus
• ASTM B368 - Test Method for Copper-Accelerated Acetic Acid-Salt Spray (Fog) Testing (CASS Test)
• ASTM D1748 - Standard Test Method for Rust Protection by Metal Preservatives in the Humidity Cabinet
• SAE J2334 – Laboratory Cyclic Corrosion Test

Using these above standards, engineering professionals can design their accelerated corrosion experiments with standardized conditions, and include specifications for evaluation criteria and evaluation durations which meet the engineering objectives of the materials and surfaces.