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Stainless Steel – Corrosion Resistance
March 12, 2025
Stainless steels are iron-based alloys that contain a minimum of 10.5% chromium. The chromium results in the formation of a thin layer of chromium oxide on the surface of the steel. This thin layer of oxide protects the metal underneath from corrosive environments, giving stainless steels their corrosion resistance. If the oxide layer is damaged there is rapid regeneration of the layer, thus preserving the corrosion resistance.
Many stainless steel alloys are available. The main distinctions between the different alloys are the metallurgical phase(s) present, corrosion resistance, and strength. The metallurgical phase(s) present largely depend on the alloy composition. Adding more chromium, above the minimum 10.5 %, increases corrosion resistance, as does adding manganese, nickel, molybdenum, or nitrogen. Strength is a function of the metallurgical phase present, whether an alloy can be cold worked and/or heat treated for strengthening, and the presence of alloying elements such as carbon, manganese, and nitrogen.
Stainless steel families
Stainless steels fall into five general classes: austenitic, ferritic, martensitic, duplex, and precipitation hardened. The distinction between each class is based primarily on the predominant phase present in the stainless steel, which is determined primarily by the alloying elements present in the alloys. The austenitic, ferritic, and martensitic alloys consist of austenite, ferrite, and martensite, respectively. The duplex alloys consist of a 50-50 mix of ferrite and austenite. The precipitation hardened alloys consist of martensite with a uniform distribution of a precipitate phase throughout the martensite.
The major alloying elements in stainless steels are chromium and nickel. Chromium primarily provides corrosion resistance and additional strength. Nickel provides strength and some corrosion resistance. Minor alloying elements include manganese, carbon, and molybdenum. Manganese is present in steels in small quantities, but at higher concentrations it stabilizes austenite and partially replaces nickel in the 200-series steels. Carbon is an impurity in austenitic steel, but is a strengthening element in ferritic and martensitic steels. Molybdenum provides additional strength and resistance to chloride pitting. Other elements, such as titanium or niobium, serve other purposes specific to the application for which the alloy was developed.
400-series steels were the first versions of stainless steel. They include the ferritic and martensitic grades that contain only chromium as a major alloying element, making them less expensive than austenitic grades. They are magnetic and generally less resistant to chloride attack than 300-series alloys. Martensitic alloys are produced by using a through hardening heating treatment with an alloy that initially consists of ferrite. The heat treatment is similar to the quench and temper process used with carbon and low-alloy steel.
Type 410 contains about 12% chromium. The ability to strengthen this alloy by heat treating to form martensite makes it a martensitic grade. Its low chromium content provides modest corrosion resistance. Given enough time, exposure to weather will cause it to rust. Type 430 is a ferritic grade that contains about 17% chromium. It cannot be strengthened by heat treating.
With sufficient quantities of nickel, stainless steel remains austenite at room temperature, creating the austenitic steels. They are nonmagnetic and cannot be heat treated for strengthening or increasing hardness like carbon steels because the phase transformation to martensite does not occur in these alloys. The primary reason for their use is their superior resistance to corrosion in the atmosphere and aggressive chemical environments compared to 400-series.
300-series alloys contain chromium and nickel, and are the most popular austenitic grades. Types 301 and 304 are the most common alloys in use and are for general use. They contain 18% chromium, 9% to 10% nickel, and up to 0.15% carbon (301) or 0.08% carbon (304) as an impurity. Other 300-series alloys are modified versions of these alloys to achieve specific properties. Type 316 contains 2%-3% molybdenum to improve the resistance to corrosion in chloride-containing environments.
Types 304L, 316L and other L-grades contain reduced carbon, less than 0.03%, to avoid microstructure changes during welding and other thermal processes which can damage the corrosion resistance. This detrimental change is known as sensitization. Types 321 and 347 contain small amounts of titanium and niobium, respectively, to prevent sensitization. They are capable of service at elevated temperatures, while the L-grades are intended to resist sensitization during fabrication. The image below shows the austenite grains in a 304 alloy.
200-series steels are also austenitic. These alloys have manganese substituted for some of the nickel as a cost-saving measure. Grade 201 contains about 17% chromium, 6.5% manganese, and 4% nickel. It has corrosion resistance similar to 301.
Precipitation hardening (PH) steels are strengthened by heat treating to form precipitates, as well as by martensite formation. They can be strengthened to higher hardness than 400-series grades by an aging method similar to that of aluminum alloys. 17-4 PH and 17-7 PH steels contain 17% chromium and 4% or 7% nickel, respectively. Minor alloying elements can include copper, titanium and niobium, and others.
Duplex stainless steels allow savings in material costs in corrosive applications such as chemical processing, including chloride- and sulfur-bearing environments. They consist of a mixture of austenite and ferrite in roughly equal proportions. Duplex stainless steels are subdivided into lean, standard, super, or hyper duplex based on the quantity of alloying elements. Duplex stainless steels contain more chromium and less nickel than 300-series and typically include nitrogen as an additional austenite stabilizer and molybdenum for corrosion resistance. 2205 (22% chromium, 5% nickel, and 3% molybdenum) is a common standard duplex stainless steel, and 2507 (25% Cr, 7% Ni plus 4% Mo) is a common super-duplex steel. The micrograph below shows a duplex stainless steel.
The broad array of stainless steels available provides a vast portfolio of capabilities. However, each alloy has distinct advantages and disadvantages. When selecting a grade of stainless steels available it is important to consider how components will be fabricated and joined together, the specific environment to which it will be exposed, in addition to the considerations common to other alloys such as mechanical requirements and cost.
Written by Michael Pfeifer, Ph.D., P.E.