Why stainless steel corrodes

Why stainless steel corrodes

  Why stainless steel corrodes

    Stainless steel has unique properties which can be taken advantage of in a wide variety of applications in the construction industry. This paper reviews how research activities over the last 20 years have impacted the use of stainless steel in construction. Significant technological advances in materials processing have led to the development of duplex stainless steel tubing with excellent mechanical properties; important progress has also been made in the improvement of surface finishes for architectural applications Structural research programmes across the world have laid the ground for the development of national and international specifications, codes and standards spanning both the design, fabrication and erection processes. Recommendations are made on research activities aimed at overcoming obstacles to the wider use of stainless steel in construction. New opportunities for stainless steel arising from the shift towards sustainable development are reviewed, including its use in nuclear containment structures, thin-walled cladding and composite floor systems.

    Stainless steels are used in countless diverse applications for their corrosion resistance. Although they have extremely good general resistance, they are nevertheless susceptible to pitting corrosion. This localized dissolution of an oxide-covered metal in specific aggressive environments is one of the most common and catastrophic causes of failure of metallic structures. The pitting process has been described as random, sporadic and stochastic and the prediction of the time and location of events remains extremely difficult1. Many contested models of pitting corrosion exist, but one undisputed aspect is that manganese sulphide inclusions play a critical role. Indeed, the vast majority of pitting events are found to occur at, or adjacent to, such second-phase particles2,3. Chemical changes in and around sulphide inclusions have been postulated4 as a mechanism for pit initiation but such variations have never been measured. Here we use nanometre-scale secondary ion mass spectroscopy to demonstrate a significant reduction in the Cr:Fe ratio of the steel matrix around MnS particles. These chromium-depleted zones are susceptible to high-rate dissolution that ‘triggers’ pitting. The implications of these results are that materials processing conditions control the likelihood of corrosion failures, and these data provide a basis for optimizing such conditions.

    Stainless steel remains stainless, or does not rust, because of the interaction between its alloying elements and the environment. Stainless steel contains iron, chromium, manganese, silicon, carbon and, in many cases, significant amounts of nickel and molybdenum. These elements react with oxygen from water and air to form a very thin, stable film that consists of such corrosion products as metal oxides and hydroxides. Chromium plays a dominant role in reacting with oxygen to form this corrosion product film. In fact, all stainless steels by definition contain at least 10 percent chromium.

    The presence of the stable film prevents additional corrosion by acting as a barrier that limits oxygen and water access to the underlying metal surface. Because the film forms so readily and tightly, even only a few atomic layers reduce the rate of corrosion to very low levels. The fact that the film is much thinner than the wavelength of light makes it difficult to see without the aid of modern instruments. Thus, although the steel is corroded on the atomic level, it appears stainless. Common inexpensive steel, in contrast, reacts with oxygen from water to form a relatively unstable iron oxide/hydroxide film that continues to grow with time and exposure to water and air. As such, this film, otherwise known as rust, achieves sufficient thickness to make it easily observable soon after exposure to water and air.

    In summary, stainless steel does not rust because it is sufficiently reactive to protect itself from further attack by forming a passive corrosion product layer. (Other important metals such as titanium and aluminum also rely on passive film formation for their corrosion resistance.) Because of its durability and aesthetic appeal, 304 stainless steel 3 inch pipe is used in a wide variety of products, ranging from eating utensils to bank vaults to kitchen sinks.

    Completely and infinitely recyclable, stainless steel is the “green material” par excellence. In fact, within the construction sector, its actual recovery rate is close to 100%. Stainless steel is also environmentally neutral and inert, and its longevity ensures it meets the needs of sustainable construction. Furthermore, it does not leach compounds that could modify its composition when in contact with elements like water.
    In addition to these environmental benefits, 304 stainless steel round tube is also aesthetically appealing, extremely hygienic, easy to maintain, highly durable and offers a wide variety of aspects. As a result, stainless steel can be found in many everyday objects. It also plays a prominent role in an array of industries, including energy, transportation, building, research, medicine, food and logistics. 

    Stainless steel is a type of steel alloy containing a minimum of 10.5% chromium. Chromium imparts corrosion resistance to the metal. Corrosion resistance is achieved by creating a thin film of metal oxides that acts as protection against corrosive materials. A popular grade of stainless steel is stainless steel 316. Stainless steel 316 is generally composed of 16 – 18% chromium, 10 – 14% nickel, 2 – 3% molybdenum, and about 0.08% carbon. The added molybdenum makes this grade more corrosion resistant than the other types. Aside from those mentioned, other elements can be added to modify certain properties of the alloy. Stainless steel 316 is widely used in highly corrosive environments such as chemical plants, refineries, and marine equipment.

    Stainless steel 316L has a lower carbon content and is used in applications that subject the metal to risks of sensitization. The higher carbon variant is stainless steel 316H which offers greater thermal stability and creep resistance. Another widely used grade of stainless steel 316 is the stabilized 316Ti. Stainless steel 316Ti offers better resistance to intergranular corrosion.

    Stainless steel utilizes the principle of passivation wherein metals become "passive" or unreactive to oxidation from corrosive compounds found in the atmosphere and process fluids. Passivation is done by allowing the stainless steel to be exposed to air where it builds chromium oxides on its surface. To enhance the formation of the passive film, the alloy is introduced to a chemical treatment process where it is thoroughly cleaned by submerging it in acidic passivation baths of nitric acid. Contaminants such as exogenous iron or free iron compounds are removed to prevent them from interfering in creating the passive layer. After cleaning with an acidic bath, the metal is then neutralized in a bath of aqueous sodium hydroxide. Descaling is also done to remove other oxide films formed by high-temperature milling operations such as hot-forming, welding, and heat treatment.

    Stainless steels are available in various grades that are used for specific applications. Different grades have their degree of corrosion resistance, strength, toughness, high and low-temperature performance. Stainless steel grades are generally classified according to their microstructure. There are five main groups of stainless steel. These are austenitic, ferritic, martensitic, duplex, and precipitation hardening.

    Austenitic Stainless Steels: These are the largest group of stainless steels which comprise around two-thirds of all 304l stainless steel tube production. Their austenitic microstructure allows them to be tough and ductile, even at cryogenic temperatures. Moreover, they do not lose their strength when subjected to high temperatures. These attributes result in excellent formability and weldability. Since the austenitic structure is maintained at all temperatures, they do not respond to heat treatment. Their hardness and high tensile strength are acquired through cold working. Austenitic stainless steels are further divided according to the austenite forming elements.

    Stainless Steel 300 Series: These are stainless steels that achieve their austenitic microstructure through the addition of nickel. These are the largest subgroup and are considered general-purpose stainless steels. This sub-group includes stainless steel 316 and other popular grades such as 302, 304, and 317.

    Stainless Steel 200 Series: These are austenitic stainless steels that use manganese and nitrogen to minimize the use of nickel. Alloying with nitrogen increases their yield strength by approximately 50% than 300 series stainless steel. However, lowering the nickel content reduces the corrosion resistance of the alloy.
    Ferritic Stainless Steels: As the name suggests, these are stainless steels that have a ferritic microstructure. Its ferritic microstructure is present at all temperatures due to the addition of chromium with little or no austenite forming elements such as nickel. Because of this constant microstructure, like the austenitic stainless steel, they do not respond to heat treatment. They are more difficult to weld due to excessive grain growth and intermetallic phase precipitation, especially at higher chromium content. The result is lower toughness after welding which makes them unsuitable for structural materials. Ferritic stainless steels are designated as AISI 400 series. This designation is shared with martensitic stainless steels.

    Martensitic Unit Cell: These stainless steels have higher amounts of carbon that promotes a martensitic microstructure. Martensitic stainless steels are hardenable by heat treatment. When heated above its curie temperature, they have an austenitic microstructure. From an austenitic state, cooling rapidly results in martensite while cooling slowly promotes the formation of ferrites and cementite. Varying the carbon content results in a wide range of mechanical properties which makes them suitable for engineering steels and tool steels. Increasing the carbon content makes the stainless steel harder and stronger while decreasing it makes the alloy more ductile and formable. However, adding more carbon results in lower chromium to maintain a martensitic microstructure. Thus, higher strength is attained at the expense of corrosion resistance. They generally have lower corrosion resistance than ferritic and austenitic 316 stainless pipe.

    Duplex Stainless Steels: This type of stainless steel consists of a combination of austenitic and ferritic metallurgical structures, usually in equal amounts. It is created by adding more chromium and nickel to a standard martensitic stainless steel which promotes a duplex ferritic-austenitic microstructure. Since they do not have a constant ferritic and austenitic microstructure, they respond to heat treatment. Austenitic stainless steel is far superior to ferritic in terms of corrosion resistance and mechanical properties. However, they are highly susceptible to stress corrosion cracking. Stress corrosion cracking happens when a crack propagates when the material is subjected to a highly corrosive environment. This can lead to sudden failure of ductile materials. A ferritic microstructure is resistant to stress corrosion cracking. By combining the ferritic phase with the austenitic phase, added resistance to stress corrosion cracking is obtained. Aside from improved corrosion resistance and mechanical properties, the price of duplex stainless steels is more stable than austenitic. This is attributed to the lower nickel content. The most common grade is the standard duplex 2205. Duplex stainless steels are not covered by AISI designation.

    Precipitation Hardening Stainless Steels: These are stainless steels that can further be modified by precipitation hardening. Initially, precipitation hardening stainless steels are supplied in a solution annealed condition. Manufacturers can perform an additional aging process to attain the desired mechanical properties. Note that this heat treatment has a different mechanism than hardening martensitic stainless steels. In precipitation hardening, precipitates or secondary phase particles are allowed to form at elevated temperatures usually lower than the curie temperature. The formation of these secondary phase particles is promoted by alloying elements such as copper, niobium, aluminum, and titanium. Their growth rate, size, and dispersion are controlled by temperature and time. These secondary phase particles act as dislocation sites to the crystal structure which improves the overall toughness and strength of the metal. Moreover, they have comparable corrosion resistance with austenitic and ferritic stainless steels, unlike the martensitic varieties.