The myth of stainless steel

Hot petroleum products, steam or combustion gases – in many critical environments, stainless steel is the material of choice thanks to its properties. The material is considered robust, durable and resistant to the elements. At the same time, stainless steel is conductive, temperature-resistant and ideal for environments where hygiene is paramount. Added to this is its almost legendary reputation for corrosion resistance.

Not least its outstanding material properties – above all its corrosion resistance – make stainless steel a sought-after material for many industries. But what exactly is stainless steel? And what is the reality regarding its corrosion resistance? A look at the relevant literature makes it clear that, according to the EN 10020 standard, stainless steel is a designation for alloyed or unalloyed steels with a specific degree of purity.

These may include, for example, steels whose sulphur and phosphorus content does not exceed 0.025 per cent by mass. So-called steel alloying elements are generally responsible for their use in high-tech applications or similar environments. Some examples:

Chromium (Cr) for hardness and corrosion resistance
Cobalt (Co) for wear resistance
Manganese (Mn) for wear resistance
Molybdenum (Mo) for tensile strength
Niobium (Nb) for elasticity
Vanadium (V) for elasticity
Tungsten (W) for heat resistance

Where there is a high proportion of alloying elements, the term ‘high-alloy corrosion-resistant stainless steels’ is used in accordance with EN 10088.

Stainless steel – the myth of rust-free steel

It follows that the term ‘stainless steel’ is not synonymous with ‘rust-proof steel’. Consequently, the common practice of equating stainless steel with rust-proof steel is incorrect. To put it simply: can stainless steel rust? The answer is: yes! Contrary to popular belief, corrosion is also an issue with stainless steels. The deterioration of a material in a corrosive environment is divided into two categories:

Wet corrosion
High-temperature corrosion

Wet corrosion

Wet corrosion takes various forms. It generally occurs in conjunction with liquids containing electrolytes. These can include mineral acids, seawater or other chloride-containing media. The main forms of wet corrosion are:

General corrosion
General corrosion is often also referred to as uniform corrosion. It describes the uniform loss of metal across a surface. Corrosion rates are expressed as mass loss per unit area and time (e.g. millimetres per year). Uniform corrosion is usually caused by inorganic/organic acids and alkaline solutions.

Inorganic acids	Organic acids	Alkaline solutions
Sulphuric acid (H₂SO₄)	Formic acid (HCOOH)	Sodium hydroxide (NaOH)
Phosphoric acid (H₃PO₄)	Tartaric acid
Hydrochloric acid (HCl)	Acetic acid (CH₃COOH)
Nitric acid (HNO₃)	Lactic acid

Various alloying elements have a significant influence on general corrosion. For example, chromium (Cr), molybdenum (Mo) and copper (Cu) are capable of reducing the corrosive effects of acids. The same applies to chromium (Cr) and silicon (Si) in the case of oxidising acids. In contrast, molybdenum (Mo), carbon (C) and phosphorus (P) have a negative effect on uniform corrosion in oxidising acids. Chromium (Cr) and nickel (Ni) promote corrosion resistance in alkaline solutions.

Crevice corrosion

Crevice corrosion is a localised phenomenon. It occurs, for example, in chlorine-containing solutions in conjunction with a crevice. Crevice corrosion is frequently observed in seawater applications. The alloying elements chromium (Cr) +, molybdenum (Mo) +++ and nitrogen +++ have a protective effect against crevice corrosion.

Erosion corrosion

The combination of corrosive solutions and mechanical wear can lead to erosion corrosion. The main cause is solid particles that accumulate in the medium.

Galvanic corrosion

Galvanic corrosion occurs when different metals are electrically connected in a corrosive environment. However, galvanic corrosion rarely occurs between two types of stainless steel.

Pitting corrosion

The term ‘pitting corrosion’ refers to corrosion of stainless steel that causes spots or holes. It usually occurs in areas where the passive layer is weakened or damaged. Chloride content, pH value and temperature are usually the causes of pitting corrosion. The following alloying elements increase the corrosion resistance of stainless steel:

Chromium (Cr) +
Molybdenum (Mo) +++
Nitrogen (N) +++

High-temperature corrosion

Chemical influences from gases, slags and molten salts or metals at temperatures above 400 degrees Celsius can cause corrosion in high-alloy stainless steels. Examples of high-temperature corrosion include:

Chlorination
Carburisation
Flue gas and deposit corrosion
Sulphidation
Oxidation
Coarse grain formation/microstructural change

Key properties of the various grades of stainless steel

Resistance to wet corrosion				Mechanical properties		High-temperature properties		Processing				Physical properties
Steel grades		Uniform corrosion	Pitting/ crevice corrosion	Stress corrosion	Intergranular corrosion	Strength	Wear resistance	Oxidation/ Corrosion	Strength	Machining	Forming	Deep drawing	Welding	Thermal expansion	Thermal conductivity	Thermal properties	Electrical resistance	Magnetisation
Ferritic	Cr	E-M	E-M	H	E-M	M	M	H	L	H	L	H	L-M	L	M	M	M	Y
HT grades	––––	––––	––––	––––	M	M	H	L	H	L	H	L-M	L	M	M	M	Y
Martensitic	B	B	H	B	H	H	––––	––––	L-M	L	––––	L	L	M	M	M	Y
Duplex	Lean Duplex	M	M	H	H	H	H	––––	––––	H	L-M	L	M	M	L	L	M	Y
Duplex	H	H	H	H	H	H	––––	––––	L	L-M	L	M	M	L	L	M	Y
Superduplex	H	H	H	H	H	H	––––	––––	L	L	L	M	M	L	L	M	Y
Austenitic	Cr-Mn	B	B	B	B	M-C	M	B	M	M	H	M-H	H	––––	L	L	M	N
CR-Ni	B	B	H	H	M-C	M	M-H	M	M	H	M-H	H	H	L	L	M	N
CR-NI-Mo	M	M	H	H	M-C	M	E-M	M-H	M	H	M-H	H	H	L	L	M	N
High Performance	H	H	H	H	M-H	M-H	––––	––––	L	M-H	M-H	M	H	L	L	M	N
HAT grades	––––	––––	––––	––––	M	H	H	H	M	H	––––	M	H	L	L	M	N

B = basic; M = moderate; L = low; H = high; Y = yes; N = no; C = cold-hardened
Source: OUTOKUMPU

To get a rough idea of corrosion resistance, it is useful to divide the grades into three simple corrosion classes:

Low
Moderate
High

Alternative grades of stainless steel for different corrosion classes

Corrosion class	Ferritic	Martensitic	Duplex	Austenitic
Basic	1.4003, 1.4512 …	1.4006, 1.4005, 1.4021, …
	1.4016, …	1.4313, 1.4542, …		1.4618, 1.4372
	1.4509, 1.4510, …	1.4122, 1.4376		1.4301 / 1.4307, 1.4541, 1.4306, …
Moderate	1.4521		1.4162, 1.2304	1.4401/1.4404, 1.4571
				1.4436/1.4432, …
High				1.4438, 1.4439
			1.4462, 1.2205	1.4466, 1.4439
			1.2507, 1.4501	1.4547, 1.4529, 1.4565
				1.4652

Stainless steel – basic corrosion resistance

The ferritic microstructure makes the stainless steel magnetic and improves its resistance to stress corrosion cracking. It is well suited to deep drawing, although its stretchability is limited. The low-alloy stainless steels 1.4003 and 1.4512 offer comparatively high corrosion resistance. The non-stabilised grade 1.4016 is even more corrosion-resistant, but unlike grades 1.4512, 1.4509 and 1.4510, it is less suitable for welding thicker sections. The stabilised grades 1.4509 and 1.4510 are comparable in corrosion resistance to the austenitic grades 1.4301/1.4307.

The martensitic grades 1.4006 and 1.4005 are generally not used where corrosion resistance is required. However, with grades 1.4313 and 1.4542, there are also options here to ensure corrosion resistance.

The austenitic grade 1.4372 offers a cost advantage compared to grades 1.4301/1.4307, although its corrosion resistance is slightly lower. Delayed cracking may occur following demanding forming operations. Grades 1.4301/1.4307 are among the most widely used stainless steels worldwide. They are easily formable and can be welded without difficulty. At the same time, they are susceptible to stress corrosion cracking.

Stainless steel – moderate corrosion resistance

The double-stabilised ferritic grade 1.4521 offers corrosion resistance that is almost on a par with grades 1.4401/1.4404 – but provides better resistance to stress corrosion cracking. 1.4521 is magnetic and is suitable for deep drawing.

The lean duplex grade1.4162 has higher corrosion resistance than grades 1.4301/1.4307 and good resistance to stress corrosion cracking. Machinability is good, but formability is more difficult. Welding is more challenging than with grade 1.4307. The corrosion resistance of grade 1.4362 is at least on a par with that of grades 1.4401/1.4404, whilst offering higher resistance to stress corrosion cracking. Mechanical formability is higher than for grade 1.4404. In addition, welding is more demanding.

Austenitic stainless steels in the medium corrosion resistance class, such as 1.4401/1.4404, are very popular. A higher molybdenum content makes the 1.4301/1.4307 grade even more corrosion-resistant. For the same reason, the corrosion resistance of the 1.4436/1.4432 grades is surpassed once again. Formability and weldability are good. However, sensitivity to stress corrosion cracking is evident in certain environments.

Stainless steel – high corrosion resistance

Duplex grades with high corrosion resistance include 1.4462 and 1.4501 / 1.4410. They generally offer better corrosion resistance than the austenitic grades 1.4438 and 1.4439. In addition, they offer greater resistance to stress corrosion cracking, as well as high mechanical strength and good abrasion resistance. However, they are more challenging to form and weld than type 1.4404.

The rule of thumb here is: the higher the alloy content, the better the corrosion resistance. In the field of corrosion-resistant stainless steels, these are therefore referred to as high-alloy stainless steels. This can be identified by the ‘X’ in the type designation according to EN (1.4301 corresponds to X5CrNi 18-10).

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