Corrosion is a major problem with most metals. It is the gradual degradation/deterioration of a metal/alloy caused by chemical or electrochemical attack due to environment. Information about corrosion in stainless steel is given in this article.
Corrosion in Stainless Steels
The addition of chromium to a steel results in the formation of a thin stable, non-porous, tightly adherent and ductile layer of primarily chromium oxide on the surface of the steel provided that it is exposed to air or another oxidizing environment. Since this layer conveys passivity to the steel, which means that it does not actively corrode, it is also called a passive layer. It is responsible for the ability of the steel to resist corrosion. The thickness of this very thin layer is of the order of 1 – 10nm (1 nanometre = 10-9 m or 10-6 mm).
It is also possible that the passive layer is damaged by tools during manufacturing (milling, grinding, polishing, drilling, tapping) or by accident. Under normal conditions (in the presence of air or an oxidizing environment) the passive layer forms itself anew, it is self-healing. This interesting capability of stainless steel is of great practical importance as no special measures are needed to renew or repair the corrosion resisting layer.
The concept of passive film formation is important because any conditions which prevent the formation of the film or cause it to break down will also lead to loss of corrosion resistance. Corrosion in stainless steel therefore occurs if the passive film is damaged and is not allowed to re-form. The behavior of the passive film depends on the composition of the steel, its surface treatment and the corrosive nature of its environment.
Stainless steels are generally very corrosion resistant and will perform satisfactorily in most environments. The limit of corrosion resistance of a given stainless steel depends on its constituent elements which mean that each grade has a slightly different response when exposed to a corrosive environment. Care is therefore needed to select the most appropriate grade of stainless steel for a given application.
Most specifiers and designers understand the importance of selecting a grade of stainless steel, for example 1.4301 (304) or 1.4401 (316). But surface finish is at least as important. In short, a bright polished surface gives maximum corrosion resistance.
The most common reasons for a metal to fail to live up to expectations regarding corrosion resistance are:
- Incorrect assessment of the environment or exposure to unexpected conditions, e.g. unsuspected contamination by chloride ions.
- The way in which the stainless steel has been worked or treated may introduce a state not envisaged in the initial assessment.
Experience indicates that any serious corrosion problem is most likely to show up in the first two or three years of service; problems can usually be explained by defects in bringing stainless steels into operation.
In certain aggressive environments some grades of stainless steel are susceptible to corrosion. Six common corrosion mechanisms are described below.
General (uniform) Corrosion
Normally, stainless steel does not corrode uniformly as do ordinary carbon and alloy steels. However, with some chemicals, notably acids, the passive layer may be attacked uniformly depending on concentration and temperature and the metal loss is distributed over the entire surface of the steel. If stainless steel is to come into contact with chemicals, design shall be based on published data to predict the life of the component. Published data list the removal of metal over a year. Tables of resistance to various chemicals are published by various organizations and a very large collection of charts, lists, recommendations and technical papers are available though stainless steel manufacturers and suppliers.
Pitting Corrosion
Pitting is a localized form of corrosion which can occur as a result of exposure to specific environments, most notably those containing chlorides (such as sodium chloride in sea water). Chloride ions facilitate a local breakdown of the passive layer, especially if there are imperfections in the metal surface. One reason why pitting corrosion is so serious is that once a pit is initiated there is a strong tendency for it to continue to grow, even although the majority of the surrounding steel is still untouched. This is not related to published corrosion data as it is an extremely localized and severe corrosion which can penetrate right through the cross section of the component. Grades high in chromium, and particularly molybdenum and nitrogen, are more resistant to pitting corrosion.
The tendency for a particular steel to be attacked by pitting corrosion can be evaluated in the laboratory. A number of standard tests have been devised, the most common is that given in ASTM G48.
The Pitting Resistance Equivalent Number (PREN) has been found to give a good indication of the pitting resistance of stainless steels. Pitting resistance equivalent numbers (PREN) are a theoretical way of comparing the pitting corrosion resistance of various types of stainless steels, based on their chemical compositions. The PREN (or PRE) numbers are useful for ranking and comparing the different grades, but cannot be used to predict whether a particular grade will be suitable for a given application, where pitting corrosion may be a hazard.
These are 'linear' formulas, where the molybdenum and nitrogen levels are 'weighted' to take account of their strong influence on pitting corrosion resistance. The most commonly used version of the formula is,
PREN = %Cr + 3.3[%Mo] + 16[%N]
Tungsten is also included in the molybdenum-rating factor to acknowledge its affect on pitting resistance in the tungsten bearing super-duplex types (1.4501). A modified formula is then used as under.
PREN = %Cr + 3.3[%Mo +0.5(%W)] + 16[%N]
Grades with a PREN of 40 or more are known as 'super' austenitics or 'super' duplex types, depending to which basic family they belong.
PREN of some grades of stainless steels are given in the table given below.
| Grade | Type | Cr | Mo | N | PREN |
|---|---|---|---|---|---|
| Ferritics | |||||
| 1.4016 | 430 | 16.0-18.0 | NS | NS | 16.0 |
| Austenitics | |||||
| 1.4301 | 304 | 17.0-19.5 | NS | 0.11max | 17.0-20.8 |
| 1.4311 | 304LN | 17.0-19.5 | NS | 0.12-0.22 | 18.9-23.0 |
| 1.4401 | 316 | 16.5-18.5 | 2.0-2.5 | 0.11max | 23.1-28.5 |
| 1.4406 | 316LN | 16.5-18.5 | 2.0-2.5 | 0.12-0.22 | 25.0-30.3 |
| 1.4539 | 904L | 19.0-21.0 | 4.0-5.0 | 0.15max | 32.2-39.9 |
| 1.4547 | 254SMO | 19.5-20.5 | 6.0-7.0 | 0.18-0.25 | 42.2-47.6 |
| Duplex | |||||
| 1.4362 | SAF 2304 | 22.0-24.0 | 0.1-0.6 | 0.05-0.20 | 23.1-29.2 |
| 1.4462 | SAF 2205 | 21.0-23.0 | 2.5-3.5 | 0.10-0.22 | 30.8-38.1 |
| 1.4410 | SAF 2507 | 24.0-26.0 | 3.0-4.0 | 0.24-0.35 | 37.7-46.5 |
| 1.4501 | Zeron 100 | 24.0-26.0 | 3.0-4.0 | 0.2-0.3 | 37.1-44.0 |
NS – Not specified
In the above table, use of trade names (name of manufacturers) is as under.
| Trade Name | Manufacturer |
|---|---|
| 904L, 254SMO | AvestaPolarit Ltd |
| SAF 2304, SAF 2205, SAF 2507 | Sandvik Steel UK |
| Zeron 100 | Weir Materials Limited |
Crevice Corrosion
The corrosion resistance of a stainless steel is dependent on the presence of a protective oxide layer on its surface. It is possible under certain conditions for this oxide layer to break down, for example in reducing acids. In such case, stainless steel requires supply of oxygen to make sure that the passive layer can form again.
Crevice corrosion is a localized form of attack which is initiated by the extremely low availability of oxygen in a crevice. It is only likely to be a problem in stagnant solutions where a build-up of chlorides can occur. The severity of crevice corrosion is very dependent on the geometry of the crevice; the narrower (around 25 micro-meters) and deeper the crevice, the more severe the corrosion. Crevices typically occur between nuts and washers or around the thread of a screw or the shank of a bolt. Crevices can also occur under deposits on the steel surface.
Crevice corrosion is a very similar mechanism to pitting corrosion. Alloys resistant to one are generally resistant to both. Crevice corrosion can be viewed as a more severe form of pitting corrosion as it will occur at significantly lower temperatures than does pitting. Crevice corrosion is avoided by sealing crevices with a flexible sealant or by using a more corrosion resistant grade.
Stress Corrosion Cracking (SCC)
Under the combined effects of stress, temperature and certain corrosive environments stainless steels can be subject to this form of corrosion. The stresses must be tensile, but do not need to be very high in relation to the proof stress of the material and can result from loads applied in service, or stresses set up by the type of assembly e.g. interference fits of pins in holes, or from residual stresses resulting from the method of fabrication such as cold working, welding or bending. The most damaging environment is a solution of chlorides in water such as sea water, particularly at elevated temperatures. As a consequence stainless steels are limited in their application for holding hot waters (above about 50°C) containing even trace amounts of chlorides (more than a few parts per million). This form of corrosion is only applicable to the austenitic group of steels and is related to the nickel content. Grade 316 is not significantly more resistant to SCC than is 304. The duplex stainless steels are much more resistant to SCC than are the austenitic grades, with grade 2205 being virtually immune at temperatures up to about 150°C, and the super duplex grades are more resistant again. The ferritic grades do not generally suffer from this problem at all.
Another form known as sulphide stress corrosion cracking (SSCC) is associated with hydrogen sulphide in oil and gas exploration and production. A threshold stress can sometimes be identified for each material – environment combination. Some published data show a continuous fall of threshold stress with increasing H2S levels. To guard against SSCC, NACE specification MR0175 for sulphide environments limits the common austenitic grades to 22HRC maximum hardness.
Bimetallic (galvanic) corrosion
Bimetallic (galvanic) corrosion may occur when dissimilar metals are in contact in a common electrolyte (e.g. rain, condensation etc.). If current flows between the two, the less noble metal (the anode) corrodes at a faster rate than would have occurred if the metals were not in contact. The rate of corrosion also depends on the relative areas of the metals in contact, the temperature and the composition of the electrolyte.
If the area of the less noble material (the anodic material) is large compared to that of the more noble (cathodic material) the corrosive effect is greatly reduced, and may in fact become negligible. Conversely a large area of noble metal in contact with a small area of less noble will accelerate the galvanic corrosion rate.
Bimetallic corrosion may be prevented by excluding water from the detail (e.g. by painting or taping over the assembled joint) or isolating the metals from each other (e.g. by painting the contact surfaces of the dissimilar metals). Isolation around bolted connections can be achieved by non-conductive plastic or rubber gaskets and nylon or teflon washers and bushes.
Intergranular Corrosion
This is now quite a rare form of corrosion. When austenitic stainless steels are subject to prolonged heating between 450-850° C, the carbon in the steel diffuses to the grain boundaries and precipitates chromium carbide. This removes chromium from the solid solution and leaves a lower chromium content adjacent to the grain boundaries. Steels in this condition are termed 'sensitized'. The grain boundaries become prone to preferential attack on subsequent exposure to a corrosive environment. This phenomenon is known as weld decay when it occurs in the heat affected zone of a weldment. It should be noted that carbide precipitation depends upon carbon content, temperature and time at temperature. The most critical temperature range is around 700°C, at which 0.06% carbon steels will precipitate carbides in about 2 minutes, whereas 0.02% carbon steels are effectively immune from this problem. Thus the problem is avoided by choosing a low carbon grade the so-called ‘L’ grades which have low carbon content (~0.03%) or by using steel with Titanium or Niobium which preferentially combines with Carbon, leaving the chromium in solution and ensuring full corrosion resistance. These are the stabilized grades 321 (stabilized with titanium) and 347 (stabilized with niobium).
Importance of Segregating Carbon and Stainless Steel
Sometimes "rusting" of stainless steel is due to contamination. It is the rusting of carbon steel which has contaminated the surface of the stainless steel at some point in the production process. Possible sources of contamination from carbon steel are tools, lifting gear (ropes, chains), grinding dust, cutting sparks, wire brushes, etc.
Wherever possible, stainless steel and carbon steel should be fabricated in separate areas of the workshop or better still in separate workshops. Where this is not possible, it is important to clean down machines used for carbon steel before using them for stainless steel. Stainless steel surfaces should be protected with plastic coatings for as long as possible during storage.
In short, it is necessary to avoid contamination of the surface of stainless steel components by carbon steel at all stages of fabrication, handling, storage, transportation and erection.
Selection of Stainless Steels for Corrosion Resistance
Chromium (Cr) content sets stainless steels apart from other steels. Commercially available grades have around 11% chromium as a minimum. These can be either ferritic or martensitic, depending on carbon range control.
Increasing chromium enhances corrosion and oxidation resistance, so a 17% Cr 430 (1.4016) ferritic would be expected to be an improvement over the '410S' (1.4000) types. Similarly martensitic 431 (1.4057) at 15% Cr can be expected to have better corrosion resistance than the 12% Cr 420 (1.4021 / 1.4028) types.
Chromium levels over 20% provide improved 'aqueous' corrosion resistance for the duplex and higher alloyed austenitics and also forms the basis of the good elevated temperature oxidation resistance of ferritic and austenitic heat resisting grades, such as the quite rare ferritic 446 (25% Cr) or the more widely used 25 % Cr, 20% nickel (Ni) austenitic 310 (1.4845) grade.
In addition to this basic 'rule', nickel (Ni) widens the scope of environments that stainless steels can 'handle'. The 2% Ni addition to the 431 (1.4057) martensitic type improves corrosion resistance marginally. Additions of between about 4.5% and 6.5% Ni are made in forming the duplex types. The austenitics have ranges from about 7% to over 20%.
However, the corrosion resistance is not simply related to nickel level. It would be wrong to assume that a 304 (1.4301) with its 8% Ni therefore has better corrosion resistance than S32205 (1.4462) duplex with only 5% Ni.
More specific alloy additions are also made with the specific aim of enhancing corrosion resistance. These include molybdenum (Mo) and nitrogen (N) for pitting and crevice corrosion resistance. The 316 types are the main Mo bearing austenitics. Many of the currently available duplex grades contain additions of both Mo and N.
Copper is also used to enhance corrosion resistance in some 'common', but hazardous, environments such as 'intermediate' concentration ranges of sulphuric acid. Grades containing copper include the austenitic 904L (1.4539) type.
Detecting Susceptibility to Intergranular Attack
Susceptibility to intergranular attack in austenitic stainless steels can be detected by performing test as per ASTM A262. This specification covers the standard practices for detecting susceptibility to intergranular attack in austenitic stainless steels. These practices include five intergranular corrosion tests, namely: (1) oxalic acid etch test for classification of etch structures of austenitic stainless steels, (2) ferric sulfate-sulfuric acid test, (3) nitric acid test and (4) copper-copper sulfate-sulfuric acid test for detecting susceptibility to intergranular attack in austenitic stainless steels; and (5) copper-copper sulfate-50% sulfuric acid test for detecting susceptibility to intergranular attack in molybdenum-bearing cast austenitic stainless steels. For more information please refer the specification.
