It is generally preferred that the electrical enclosures are manufactured from stainless steel material. Stainless steel enclosure are used in various industrial applications where corrosion resistance is important. However, there are rules to be considered when selecting and using stainless steel materials.
Which grade of stainless steel?
304 grade stainless steel is the most widely used type in different applications where a special corrosion resistance is not required. More harsh environments require the use of 316 qualities which is more expensive grade of stainless steel. However, even for 316 grade stainless steel, chemicals, their concentration and their level of aggressiveness in the environment must be examined, evaluated and observed in deep detail; because there may be cases where this grade is not sufficient.
304 Grade Stainless Steel
SEA 304-304L / EN 1.4301-1.4303
Classified as 304 grade by the American Society of Automotive Engineers – SAE, stainless steel contains 18-20% chromium and 8-10.5% nickel. The 304 grade sometimes referred to as ‘18 -8 ‘, is widely used as standard stainless steel grade in different industrial applications. The most common application where the use of 304 quality stainless steel is inconvenient, is the outdoor applications in the coastal areas where the salt is deposited on the surfaces by flying. In the food and beverage industry, it is necessary for the enclosures to be washed frequently without worrying about rusting. 304 grade stainless steel is used extensively here. Due to its high resistance to various acids found in meat, milk, fruits and vegetables, 304 grade stainless steel has become the ideal raw material for control and control equipment enclosures of machines used in food and beverage processes. Nevertheless, a stronger stainless steel grade may be required, if the environment has excessive chloride usage, or cleaning procedures require high corrosion resistance solvents. The use of 304 grade stainless steel may be sufficient in a majority of waste treatment and clean water treatment plants. However, in some clean water treatment plants, chlorine gas is used as a disinfectant and sulfur dioxide is used to remove the residues of used chlorine. When the nature of the environment with high humidity and the corrosive effect of these gases are combined together, the capabilities of 304 grade stainless steel are exceeded. Some municipalities put efforts to use less chemicals with environment friendly approaches and apply ultraviolet treatment for water treatment. 304 quality stainless steel can be used, due to no chemical usage in such facilities.
316 Grade Stainless Steel
SEA 316-316L / EN 1.4401-1.4404
The second most widely used grade of stainless steel is 316 when the limits of 304 are reached. In general, 316 quality stainless steel, which is 25-35% more expensive, has a higher nickel content and an additive molybdenum content of 2-3%. Molybdenum additive provides increase of corrosion resistance especially against chlorides. The rate of chloride contained in water is the most determinant factor in the selection of stainless steel enclosure material for water treatment plants. 304 grade stainless steel is able to withstand against the corrosive effect of water containing up to 100 ppm of chloride. On the contrary, in the 316 grade stainless steel material, this limit is up to 1000 ppm of chloride. In addition to chlorides, there are other substances that affect the corrosive behavior of the water. Especially, attention should be paid to halides such as bromides or iodides which are not chlorides. Another important factor is the presence of water oxidizers (e.g. ozone); the risk of point corrosion is also increasing as long as water oxidizing power increases. In such some cases, 316 grade stainless steel is also
316 grade stainless steel commonly known as the marine standard, cannot withstand to be presented in continuous saline water (equivalent to 19,000 ppm chlorine). However, it can be used in overwater applications such as temporary and short-term salt spray applications. It should not be forgotten to take various precautions for the application areas with narrow and closed low airflow a difficult harsh environment conditions with low pH or high temperature.
316 grade stainless steel is generally considered as an adequate enclosure for marine applications such as ships and oil platforms and even for salt mines. Essentially, the stainless steel grade is 316 which should be used mainly in corrosive environments where high chloride levels are present.
316 grade stainless steel should be preferred because of having high chloride resistance in food processing plants using excessive acidic chemicals and concentrated chloride salts. 316 grade stainless steel having considerable degree of resistance against sulfuric acid solutions (<10%) and sulfur-containing gases, is used as the raw materials for the enclosures used in industries where they are frequently found. For example, 316 grade stainless steel is used in the paper industry. Similarly, in other industries such as vulcanization process for the rubber or gun powder manufacturing, it is also widely used.
Nitric and hydrochloric acid
As a general rule, 316 grade stainless steel enclosures are more resistant to aggressive chemical environments than 304 grade. Nitric acid is at the top of special cases which does not suit this rule. Nitric acid is especially used in fertilizer production, pharmaceutical industry and explosive production; also, it can be used in synthetic yarn and polymer manufacturing or in water treatment plants. All stainless steel grades have a slight resistance to nitric acid, but in industries where the use of nitric acid is widespread, 304 grade stainless steel should be preferred due to its high resistance against this substance. In this case, the self-passive ability of the stainless steel is enhanced by the strong oxidizing effect of nitric acid and the corrosion resistance of the stainless steel is increased. It is important to consider that neither the 304 grade nor the 316 grade stainless steel will be able to withstand against the chemical corrosion caused by hydrochloric acid.
This acid destroys the passive surface and leaves the metal vulnerable.
Swimming pool environments
The swimming pool atmosphere is generally characterized by at a relative humidity approximately 60-70% together with chlorine and chlorine compounds relatively high levels. The original “pool smell” is existent due to these. These substances can be spread to the entire space as both steam and aerosol. In addition to sodium chloride, relatively high amounts of calcium and magnesium chloride are found in many applications. Typical values are 2-5% for the chloride ratio and 3-4 for the pH value. The high relative humidity rate fluctuates by depending on the intensity of swimming activity, the temperature changes during the day
and resulting in the accumulation of deposits at various locations with the switched off air conditioning system in the nights. The evaporation of chloride-containing liquids and the increase of chloride concentration in the contact zones considerably increase the likelihood of corrosion.
It is necessary to pay special attention to the selection of the right material in swimming pool applications. Especially, attention should be paid to the selection of materials for enclosures placed in closed areas such as machine rooms of pools. Special precautions such as micro climate air quality, should be taken in these applications. It is obvious that the selection of 316 grade stainless steel will not sufficient alone in the presence of oxidizing substances such as ozone or where disinfectant chemicals are stored. In this case application of lacquer coating or 316 quality stainless steel or both is required.