Stainless steel - a versatile and worthwhile material
Definition, material characteristics and advantages of this material at a glance.
Definition of the term
Long before the turn of the last century, it was discovered that steel could be made more corrosion resistant by adding nickel and chrome. However, the new types of steel which were enriched with these elements still left much to be desired. The pivotal breakthrough was made in Germany in 1912. By combining nickel and chrome in conjunction with a precisely regulated heat treatment process, both optimum corrosion resistance and good mechanical properties were finally attained.
The refined designations of the time, V2A and V4A - "V" for experiment and "A" for austenite - are still used today as synonyms for rust-free stainless steel. Both of the two largest German producers adopted the designations Nirosta and Remanid as brand names for their products in this group. Internationally, the terms 18/10 or 18/8 are typically used, as these indicate the most common alloy ratios of chrome and nickel in stainless steel. However, standardised DIN material numbers (e.g. 1.4301) are used in industry to make precise distinctions between the various types of stainless steel.
Material characteristics and advantages
1.4301 stainless steel is an austenitic, acid-resistant 18/10 Cr-Ni steel, which exhibits better corrosion resistance due to its low carbon content. It is approved for use at temperatures up to 300 degrees Celsius. If higher working temperatures are required, titanium stabilised steel in accordance with material no. 1.4541 should be used. This steel is resistant to water, steam, humidity, table acids as well as weak organic and inorganic acids, and is suitable for a wide range of applications. For example, in the food industry, for beverage production, in the pharmaceutical and cosmetics industries, in chemical apparatus engineering and the field of medical technology.
The advantages of stainless steel at a glance:
- Temperature resistant
- Can be welded
- Low maintenance
Rust free stainless steel owes its corrosion resistance to a simple chemical reaction: An extremely thin passive layer is formed as a result of the exposure of the chrome component of the steel to the acid in air or water. This layer protects the steel from all corrosive substances. If it is ever damaged by external influences, it re-emerges from the matrix of steel in just fractions of a second. The corrosion resistance is primarily effectuated by the chrome component. Increased resistance can be achieved through enrichment with nickel and molybdenum, as well as other alloying agents. Hence the wide variety of rust-free types of stainless steel available today, which are very unique alloys formulated for specific applications.
Furthermore, corrosion resistance also depends on the surface structure. The more smooth and homogeneous the surface, the better the corrosion resistance will be. Inclusions and scaling (e.g. pressed-in rust or dust particles from processing), in particular, can cause localised corrosion which spreads out quickly.
The following types of corrosion form on stainless steel:
a) Intercrystalline corrosion
Intercrystalline corrosion develops when chromium carbides are released at the grain boundaries in critical form. This results in a chrome deficiency in the area, through which the passivating effect is lost. Material numbers 1.4541, 1.4571 and 1.4435 are qualified as resistant to intercrystalline corrosion.
b) Pitting corrosion
In pitting corrosion, the passive layer is only penetrated at specific points. This results in the emergence of pits or small holes on the surface of the metal. Pitting corrosion is largely caused by halogen ions, and particularly by chlorine ions. Above all, pitting corrosion is especially pervasive in water and around drain water, where there is often an abundance of chlorine- and chloride ions.
c) Galvanic corrosion
Galvanic corrosion is a very commonly occurring form, which arises if metallic materials of different potential come into contact where an electrolyte is present. Here the less noble metal is attacked by the electrolytes and dissolved. The intensity of the corrosion is relative to the amplitude of the current flowing in this galvanic element. We encounter galvanic corrosion on a very frequent basis. One well known example, in particular, is the combination of steel- and stainless steel flanges. It is also well established that galvanic corrosion occurs when cast iron flanges are fastened with stainless steel bolts.
d) Crevice corrosion
Crevice corrosion occurs if the passive layer of stainless steel is destroyed, for example, due to the lack of oxygen in an environment where aggressive media are present. For this reason, crevice corrosion frequently occurs in narrow crevices and small cavities (e.g. underneath gaskets or screw heads, etc). One example that should be mentioned here is that of the gas hood in digestion towers, where an aggressive media (biogas in this case) is present on the inside, where there is also a complete lack of oxygen. Here special considerations must be made to address crevice corrosion.
Cleaning and maintenance
A mild detergent is generally sufficient for the removal of fingerprints. Some detergent manufacturers offer specialised products, with which the cleaning effect is supplemented by a maintenance component. Most household cleansers are well suited for hard-to-remove dirt and stains, as it also removes limescale deposits and minor discolourations. After cleaning, the surface is rinsed with clean water. Heavy oil and grease can be removed with alcohol based cleaning agents and solvents (e.g. spirit or acetone). However, special care must be taken to prevent the dissolved contaminants from being spread over a large area of the surface. There is a special alkaline and solvent based cleaner for use on paint marks. Products containing chloride (especially those containing hydrochloric acid), bleach and silver polish should never be used.