The Selection and Care of Stainless Steel Electrical Enclosures
Electrical Enclosure material is selected based on the application, environment and location. Enclosures should be constructed of materials capable of withstanding the mechanical, electrical, thermal and environmental stresses to be encountered in service conditions. This includes protection against corrosion by use of suitable materials or by protective coatings. In this guide, we will discuss a range of important factors to consider when selecting and caring for stainless steel electrical enclosures.
Grade 304 or 316 Electrical Enclosures?
Grade 304 is the most common grade of stainless steel. Grade 304L is a low carbon version of Grade 304 and is used to mitigate risk of corrosion in welded fabrications. Grades 304 and 304L have excellent corrosion resistance properties, offering resistance to corrosion in many environments including within food and beverage processing environments. Grades 304 and 304L Electrical Enclosures are typically used within food and beverage processing facilities, and within chemical, petrochemical, mineral processing and other general industrial environments. They are higher cost than mild steel but lower cost than Grade 316. Due to increased risk of corrosion, Grades 304 and 304L Electrical Enclosures should not be used within marine and coastal saltwater environments, or within environments with temperatures above 50oC and with chlorides present.
Grade 316 is the next level up from Grade 304/304L. Grade 316 has similar characteristics to 304 but has higher corrosion resistance. Grade 316L is a low carbon version of Grade 316 used to avoid corrosion in welded fabrications. Grades 316 and 316L have excellent corrosion resistance properties. In contrast to Grades 304 and 304L, Grades 316 and 316L has increased ability to resist pitting and crevice corrosion within warm chloride environments. It is selected for more aggressive environments such as coastal environments. Grades 316 and 316L Electrical Enclosures are typically used within harsh environments including marine environments, coastal saltwater environments, food and beverage processing facilities, and within chemical, petrochemical, mineral processing and harsh industrial environments. An additional benefit of Grades 316 and 316L over 304 and 304L is that Grades 316 and 316L will remain fully non-magnetic while Grades 304 and 304L may exhibit magnetic properties as a result of the material being worked throughout the manufacturing process.
Handling and Storage of Stainless Steel Electrical Enclosures
Careful handling and storage of Stainless Steel Electrical Enclosures is important to ensure that the surface remains clean and free from defects. Care must be taken during unloading, removing from packaging, electrical assembly, transport, installation and maintenance. Stainless Steel Electrical Enclosures should be handled and stored separately from carbon steel. If placing the Electrical Enclosure on a workbench during assembly, be sure to clean the work area to ensure it is free from swarf, carbon steel filings, carbon steel dust and contaminants. It is always good practice to ensure there is a protective barrier or layer between the work surface and the stainless steel to prevent scratching and cross contamination. During electrical assembly and installation, avoid exposure to the working of carbon steel including adjacent cutting, welding, griding and sanding. Avoid exposure to moisture, chlorides, and pollutants as these can lead to corrosion, staining, and other surface damage. Scratches and dents can occur due to rough handling, sliding on work surfaces, inadequate packing, or contact with forklift tines. Use protective materials to prevent damage and cross contamination. A Stainless Steel Electrical Enclosure must be considered precision equipment and it should be handled and stored as such to ensure optimum performance throughout the lifetime of the enclosure.
Contamination and Corrosion of Stainless Steel Electrical Enclosures
Stainless Steel Electrical Enclosures are vulnerable to contamination from carbon steel and chlorides, resulting in corrosion. Avoid processing carbon steel and stainless steel in the same work area. Cutting, welding, grinding or sanding carbon steel will produce carbon contaminants that will result in cross contamination of the surface of the Stainless Steel Electrical Enclosure. In addition, the Tools used on carbon steel should not be used on Stainless Steel Electrical Enclosures. During transport, avoid direct contact between the Stainless Steel Electrical Enclosure and steel pallet strapping or tie-down straps to prevent contamination.
For installations with high levels of exposure to chemical or chemical cleaning, it is recommended to check for chemical compatibility using a chemical compatibility resistance table.
For installations in coastal saltwater environments, it is important to select suitable materials to withstand increased risk of corrosion. Surface contaminants increase the risk of corrosion. This risk increases with chloride exposure such as saltwater spray in coastal environments. In high humidity environments, above 55% RH, chlorides will cause rust spotting on the surface of the Stainless Steel Electrical Enclosure.
To reduce this risk, clean the surface of the enclosure by washing regularly with contaminant-free water and by drying the surface with a clean cloth. To provide a simple example, in the same way that a car requires a regular wash and dry to remain clean and shiny, so too does a Stainless Steel Electrical Enclosure. By performing regular maintenance of washing and drying, particularly in coastal saltwater environments, a Stainless Steel Electrical Enclosure will look brand new, clean and shiny for many years to come.
Galvanic Corrosion from Dissimilar Materials
The isolation of dissimilar materials to prevent corrosion is a well-known topic. However, there are many cases of contact between dissimilar materials that is not problematic. In fact contact between dissimilar materials is very common and even required in some instances. So when do we need to be concerned about galvanic corrosion from dissimilar materials in relation to Stainless Steel Electrical Enclosures?
For galvanic corrosion to occur, three conditions must be met:
- The dissimilar metals must be in contact with each other
- The dissimilar metals must have different potentials of sufficient value
- The join must be wet with conductive liquid
The first condition is met if the dissimilar metals are in electrical contact with each other. This connection may be via a fastener. Contact cannot occur if the surfaces are insulated from each other. However, if using an insulator, it is important to address the requirement for full earth continuity for the electrical assembly. If dissimilar materials are not insulated from each other, galvanic corrosion may occur providing the other two conditions are met.
The rate of galvanic attack is determined by the difference in potential. While sacrificial materials such as zinc have the most negative potential and noble materials such as gold has a more positive potential, stainless steel sits in the middle albeit above mild steel. The difference in potential between Grade 304 and Grade 316 is small therefore these two materials can be in contact. When dissimilar materials are in contact and wet, galvanic corrosion will occur. The sacrificial material will corrode protecting the noble material. The rate of corrosion is impacted by the surface area. If the Stainless Steel Electrical Enclosure has a large surface area in contact with the electrolyte while the sacrificial metal has a small surface area, the stainless steel will generate a large corrosion current which will be concentrated onto the sacrificial metal.
The conductive electrolyte liquid may include rainwater, water droplets from condensation, coastal saltwater spray, or other liquid from adjacent industrial processes. Saltwater spray and industrial process pollution increases the conductivity therefore galvanic corrosion is greater in these environments. To reduce the risk of the accumulation of a conductive electrolyte, particularly in coastal or highly polluted industrial environments, the Stainless Steel Electrical Enclosure may be located under a shelter, or fitted with a suitable rain hood, or coated.
Coastal Corrosion and Staining of Stainless Steel Electrical Enclosures
Discoloration of the surface of stainless steel, known as tea staining, is a cosmetic issue that occurs in coastal saltwater environments. It is cosmetic only and does not impact on the structural integrity or the life of the Stainless Steel Electrical Enclosure. Factors contributing to the severity of tea staining include atmospheric conditions and the presence of corrosive particles.
In coastal saltwater environments, high relative humidity and high temperatures provide conditions that accelerate tea staining. The increased water vapour in the atmosphere coupled with salt particle deposits settle onto the surface of the stainless steel as temperatures fall below dewpoint. Although salt water particles are the main source of tea staining, the effect is increased with the addition of industrial chemical pollutants.
AS/NZS 2312 is an Australian and New Zealand standard that provides guidelines for protecting structural steel against atmospheric corrosion. AS/NZS 2312 recognizes that in certain situations, locations 20 kilometres inland may be considered a marine environment influencing coastal corrosion. However, as a minimum, locations within 5 kilometres inland should be considered at risk of coastal corrosion.
There are several ways to reduce and eliminate the effects of tea staining.
As we have previously mentioned, Grades 304 and 304L Electrical Enclosures should not be used within marine and coastal saltwater environments, or within environments with temperatures above 50oC and with chlorides present. Grades 316 and 316L Electrical Enclosures should be used within harsh environments including marine environments, coastal saltwater environments.
It is important to ensure that water does not settle onto the surface of the Stainless Steel Electrical Enclosure. For example, water and salt deposits may settle onto the flat roof and will not drain freely. Adding a sloped roof or rain hood will promote natural drainage to reduce the risk of tea staining.
It is also important to ensure that salt particle deposits do not get trapped on the surface. A smooth surface reduces the risk of tea staining. Our range of IP55 and IP66 Stainless Steel Electrical Enclosures have a brushed finish with a surface finish of 0.4 µm Ra while our range of IP69K Enclosures, Stainless Steel Terminal Boxes and Pushbutton Enclosures have a smooth surface finish of 0.2 µm Ra. This is very smooth, resistant to salt accumulation and is easy to clean which significantly reduces the risk of tea staining.
When leaving the factory, the protective passive surface layer (the chromium oxide film) of the Stainless Steel Electrical Enclosure provides for a clean and corrosion resistant surface. The surface should be kept clean and contamination-free. If tea staining occurs, the surface should be cleaned and treated with chemical passivation such as a safe and environmentally friendly citric acid solution to restore the passivation layer.
Cleaning and Maintaining Stainless Steel Electrical Enclosures
Stainless Steel Electrical Enclosures will remain looking clean and shiny if they are washed and dried regularly.
Wash with clean water and wipe the surface dry with a new clean soft cloth. A stainless steel spray polish such as Wurth’s “Stainless Steel Care Spray” may also be used to remove residues and to renew the smooth and shiny surface. Wash and wipe the Stainless Steel Electrical Enclosure in the same direction as the grain. Wiping against the grain will spoil the finish and stainless will lose its shine.
For installations in rural and suburban areas, annual cleaning will be sufficient. For installations in industrial areas with pollutants, we recommend cleaning every 3 months as a minimum. For installations in coastal saltwater environments, cleaning should be performed every 1 to 3 months. If performing routine maintenance on surrounding equipment, consider cleaning the electrical enclosures at the same time.
Do not use scourers or steel wool. Do not use cloths that have been used on steel and do not use hydrochloric acid-based cleaning products.
Stainless Steel Fasteners
Stainless steel fasteners are used in Stainless Steel Electrical Enclosures. Galling may occur when using stainless steel bolts with stainless steel nuts. This is where the nut seizes onto the bolt during tightening or loosening. To prevent galling, keep fasteners clean and free from dirt, swarf and debris. Do not overtighten. We recommend applying a lubricant or anti-seize to the thread prior to fitting the fastener. Otherwise galling may occur and the fastener may shear when tightening.