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Corrosion of Stainless SteelTypes of corrosion, alloying elements and environmental conditions

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CQJ – Jan. 2017 Corrosion of stainless steel 1

Corrosion of Stainless Steel Types of corrosion, alloying elements and environmental conditions With regard to corrosion, stainless steel is a very smart group of metals. The excellent corrosion re-sistance combined with a (still!) affordable price has made stainless steel the most frequently used group of metals within critical sectors such as the food and pharmaceutical industries as well as the chemical industry. The excellent corrosion resistance of stainless steel is caused by a very thin layer of oxides in particu-lar chromium and iron oxides, and despite a thickness of only a few nanometers, this oxide layer is so strong that it effectively isolates the steel from the environment. Should the oxide layer suffer from a breakdown, it is quickly regenerated, and the corrosion protection is re-established. Unfortunately, this ideal scenario does not always take place; the oxide layer may be damaged with-out repassivating, and the sad result may be serious corrosion. Once the corrosion has started, rapid penetration may occur causing the stainless steel to be a very short-lived construction material. The difference in between the two extremes is sometimes very small: If repassivation takes place, corro-sion is prevented and, theoretically, the steel may last forever. If not, severe corrosion may take place, and the life-span of the equipment may be very, very short.

Fig. 1 The most commonly seen types of corrosion attacking stainless steel. General

corrosion and intergranular corrosion are rarely seen, whereas the three others are all too common. Pitting corrosion and crevice corrosion are often dealt with under the common name of ”localized corrosion”. For further information: See SS&C, Chapter 6.

Corrosion of stainless steel is discussed in detail in “Stainless Steel and Corrosion” (Claus Qvist Jessen, Damstahl a/s, October 2011), Chapter 6.

General Corrosion Intergranular Corr.

Pitting Corrosion Crevice Corrosion

Stress Corr. Cracking




General Corrosion (SS&C, Chapter 6.1) Also named acid corrosion (abtragenden Korrosion, generel korrosion), general corrosion is a type of corrosion frequently occurring in very strong acids; however, corrosion may occur in strong alkalines as well. Unlike any other type of corrosion, general corrosion is recognized by the fact that the whole oxide layer is uniformly stripped off, causing surface suffers from corrosion. The corrosion is almost uniform and expressed as a number of grams per square meter, the loss of material may be very high whereas penetration is rather slow. As mentioned above, general corrosion takes place at extreme pH values, i.e. in very strong acids or, less common, in strong alkaline solutions. Typical media are sulphuric acid, phosphoric acid and so on, and apart from the type of media and the strength, corrosion velocity is highly dependent upon the temperature and the presence of impurities, in particular chloride. As a rule, the corrosion increases with increasing temperature and increasing chloride concentration.

Fig. 2 4301 stainless steel bolt suffering from severe general corrosion after having

spent 12 weeks in a nitric acid-hydrofluoric acid (HNO3/HF) pickle bath. Please note that the loss of metal is quite uniform and quite large, while no penetration has occurred yet.

The most useful elements in the steel are nickel and molybdenum. In addition, copper (Cu) has a cer-tain beneficial effect when the stainless steel is exposed to “reducing acids”, such as sulphuric and phosphoric acids. This is utilized in the austenitic 904L (4539), which contains 1.2-2.0 % Cu. In the other side of the spectrum, low-alloyed ferritic and, in particular, martensitic steels should not be used in strong acids and alkalines. Much more detailed information regarding general corrosion of stainless steel is found in SS&C, Chapter 6.1. Pitting Corrosion (SS&C, Chapter 6.2) Pitting corrosion (Lochfraß-Korrosion, punktfrätning, grubetæring) is a type of corrosion caused by a local break-down of the protective oxide layer. Unlike the ideal situation, repassivation does not occur, and severe corrosion will take place locally, while the rest of the steel remains passive. Pitting corro-sion is the perfect example of the edge-like nature of stainless steel. Either repassivation occurs and the steel lasts forever, or corrosion takes place, and penetration may occur rapidly.




Fig. 3 Stainless steel (4301) specimen (thickness 0.5 mm) after a few days of exposure

in a dilute aqueous solution of common rock salt (NaCl) and hydrogen peroxide (H2O2). While 99 % of the steel remains unharmed, some of the pits (in the frame) have caused penetration. The microscopic photo to the right shows a magnification of the framed section. The penetration is in the center of the photo.

Usually, the risk of pitting corrosion (and also crevice corrosion, see below) increases with

Increasing chloride concentration,

Increasing temperature,

Increasing concentration of oxidants and

Decreasing pH (increasing acidity) Regarding the alloying elements, the resistance towards pitting corrosion is increasing with increasing contents of Cr, Mo and N, while the effect of Ni is comparatively small. Non-metallic impurities, such as S and P, reduce the corrosion resistance significantly. Pitting Resistance Equivalent (SS&C, Chapter 6.2.6) If we stick to the useful alloying elements, Cr, Mo and N, loads of experiments have shown that the resistance of the steel towards initiation of pitting corrosion can be estimated as a ”Pitting Resistance Equivalent”, the so-called ”PREN”. PREN is calculated according the formula below, and, as a rule of the thumb, the higher the PREN, the higher the resistance towards pitting corrosion. Two steel grades with the same PREN can therefore be expected to perform equally good towards pitting corrosion. PREN = %Cr + 3.3 · %Mo + 16 · %N Please note that molybdenum (Mo) works with a factor 3.3 and is therefore 3.3 times better than a similar amount of chromium (Cr). With a factor 16, nitrogen is even better, however, this only plays a role in duplex steels (i.e. 4462 and 4410) and high-alloyed austenites, such as 4547 (254 SMO).




Through the PREN, it’s possible to determine the resistance of a certain stainless steel towards the initiation of pitting corrosion, and it’s even possible to make a table listing the different steel grades along with their PREN. The most common grades are to be found in the Table below. For a more thorough list (including much more information dealing with the corrosion properties of stainless steel), please check out SS&C, Chapter 6.2.6, table 6.1.

Alloy (EN / popular name) Cr Mo N PREN 2.4819 / Hastelloy C-276 15 16 - 67.8 1.4547 / 254 SMO 20 6.2 0.2 43.7 1.4410 (”superduplex”) 25 4.5 0.3 43.0 1.4539 / 904L 20 4.5 - 34.5 1.4462 (UNS S32205) 22 3.0 0.15 34.3 1.4435 17.0 2.5 - 25.3 1.4436 / 4432 16.5 2.5 - 24.8 1.4362 (duplex 2304) 23 - 0.10 24.6 1.4162 (lean duplex 2101) 21 0.1 0.20 24.5 1.4401 / 4404 / AISI 316(L) 16.5 2.0 - 23.1 1.4571 / ”AISI 316Ti” 16.5 2.0 - 23.1 1.4521 / AISI 444 17 1.8 - 22.9 1.4301 / 4307 / AISI 304(L) 17.5 - - 17.5 1.4509 / AISI 441 17.5 - - 17.5 1.4016 / AISI 430 16 - - 16.0 1.4034 / AISI 440B (0,43-0,50 C) 14 0.5 - 15.7 1.4057 / AISI 431 (0,12-0,22 C) 15 - - 15.0 2.4816 / Inconel 600 14 - - 14.0 1.4021 (0,16-0,25 C) 12 - - 12.0 1.4003 / AISI 410 11 - - 11.0

Table 1 PREN table containing some of the most commonly found grades of stainless

steel. The higher the PREN, the better resistance towards pitting corrosion. The colours of the rows refer to the metallurgical structure:

Austenite Ferrite Duplex Martensite Nickel Alloys In a way, the table above can be regarded as a kind of “pecking order” for the corrosion resistance (pitting and crevice corrosion). The higher in the table, the higher the PREN, and the better is the cor-rosion resistance of the particular stainless steel. Note that the martensitic knife steels (4034, 4057 and 4021) are in the bottom, whereas the high-alloyed “super steels” (i.e. 4547, 4410) are in the top. Also note that the acid resistant 4401 has a PREN of 23.1, while the common stainless 4301 has only 17.5. This difference is the main reason why the 4401 class is better than the 4301 class in almost all media – even though these media are far from “acidic”. It’s a common misconception that “acid re-sistant” steel grades are only better in acid media. In practice, the positive effect against pitting corro-sion is far more important. Thus, the difference between 23.1 and 17.is the most important difference in between the 4301 and 4401 groups.




Finally, it’s worth noting that nickel (Ni) doesn’t play any role at all for the initiation of pitting corrosion. A very fine example is Inconel 600, a high-temperature alloy containing only 14 % Cr, but 72 % Ni, but with regards to pitting corrosion, it performs rather poorly – worse than 4301. On the other hand, the comparatively Ni is great for slowing down a propagating attack, so once the corrosion has started, Ni is quite useful. In general, though, fighting pitting corrosion is most often a question of preventing the corrosion from starting, rather than slowing down a propagating attack. Consequently, PREN be-comes a key factor of the corrosion resistance of stainless steel. The Critical Pitting Temperature (CPT, SS&C, Chapter 6.2.2)

Fig. 4 Curves showing measurements of the Critical Pitting Temperature (CPT) versus

the chloride content of the water (Cl-). The different colour curves each represent a different steel grade, and for all the area below each curve can be regarded as ”safe”. Above the curves, there is severe risk of pitting. All experiments were car-ried out at conditions resembling ”well-aerated tap water”, and note the good connection in between PREN and each curve. The higher the PREN, the higher the curve (= better steel). The difference I between the blue curve of 4301 and the red 4401 is due to the difference in PREN from 17.5 to 23.1.

A very instructive way of showing the effect of the most important environmental parameters (chloride and temperature) , as well as the corrosion resistance of the different steel grades, is obtained by measuring the so-called Critical Pitting Temperature (CPT). The Critical Pitting Temperature is the temperature at which the steel (at well-defined conditions) starts suffering from pitting corrosion. Nor-mally, such experiments are conducted in the laboratory using conditions similar to those like “well-aerated tap water”. For each experiment, the chloride content is kept constant, while the temperature

”Safe”

Pitting




is increased slowly in steps. Suddenly pitting occurs, and this is the CPT. The higher this temperature, the better is the corrosion resistance of the particular steel grade exposed to the actual conditions.. Fig. 4, above, shows the results of several series of measurements, including a number of different stainless steels, and for all, it’s worth noting the clear relationship in between the chloride and the temperature: The higher the chloride, the lower the CPT. As well, note the curves for the different steel grades: The higher the PREN (Table 1), the higher is the curve, clearly showing a better pitting corrosion resistance. Crevice Corrosion (SS&C, Chapter 6.3) Crevice corrosion (CC, Spaltkorrosion, spaltekorrosion) reminds a lot about pitting corrosion; how-ever, CC takes place in crevices, pores and narrow geometries where there is a poor exchange of media – or none at all. Such places, all transport is controlled entirely by diffusion, and, through a pe-riod of time, the medium in the crevice becomes both more acidic and contains more chloride than the “bulk media”. Therefore, the risk of corrosion in a crevice is always higher than the risk of pitting corro-sion on the ”free surfaces” outside the crevice.

Fig. 5 Weld nipple suffering from crevice corrosion on the gasket face (the red arrows).

When in operation, the gasket face has been equipped with an EPDM gasket, and underneath this gasket an aggressive environment has developed. The liq-uid responsible for the corrosion has been an aqueous solution of copper(II) chloride (CuCl2). The mechanism of CC is discussed in SS&C, Chapter 6.3.

The effect of the environmental parameters is the same for CC as for pitting corrosion, however, by experience, an old “rule of the thumb states” says that CC occurs at a temperature 20-25 ºC below that of pitting corrosion (i.e. the critical pitting temperature, CPT). Thus, all the curves in Fig. 4 should




be pushed 20-25 C downwards. If the steel is close to its corrosion limit, the equipment should be de-signed so that no crevices are present. If this is not possible, a more corrosion resistant steel must be chosen. An upgrade should be made (Table 1). Also with regards to the importance of the PREN, crevice corrosion correlates with pitting corrosion, and the values of the table above are also valid for CC. However, CC is a bit more tricky. While the comparatively noble Ni doesn’t play a role in the initiation of pitting (and thus in the PREN equation), it’s quite useful regarding repassivation, in particular in the acid, anaerobic environments which may occur in a crevice. Due to the repassivation effect of Ni, the Ni-free ferritic grades must be expected to perform slightly worse than the parallel austenites. With regards to pitting, the PREN table is usually quite valid. Stress Corrosion Cracking (SS&C, Chapter 6.4) Stress corrosion cracking (SCC, Spannungsrißkorrosion, spændingskorrosion) is a type of corrosion giving rise to cracks. SCC is the most severe type of corrosion, and penetration may occur as a mat-ter of days rather than months or years, even in thick steel plates. The name itself indicates that the corrosion takes place in regions of the steel, where tensile stress is present. Such tensile stress is common and may occur as a result of any kind of mechanical process including welding and grinding.

Fig. 6 Left: SCC in a hot-water tank made of 4301. The tank has been used for

the storage of 90 °C water; note the fine side-cracks.

Right: Micro-section through one of the cracks from the hot-water tank to the left. Note the fine appearance of the cracks, combined with the tiny side-cracks, all very typical of chloride-induced SCC. The thick-ness of the steel is 3 mm.




The risk of SCC increases with

Increasing chloride content,

Increasing temperature,

Low pH (acid conditions), and

Evaporation Of these, the temperature is, by far, the most important single factor, and SCC is more dependent on the temperature than any other type of corrosion. In addition, it’s the only type of corrosion which tends to be worse above water (evaporation) than below the water line. See SS&C, Chapter 7.4. SCC is a type of corrosion which almost selectively attacks the lowest-alloyed austenites, such as the 4301 class, and normally the 4301 grades are in danger at temperatures above 60-70 ºC. In practice, cases are known in which 4301 has been attacked by SCC at much lower temperatures, even below room temperature. Due to the content of Mo and Ni, the acid resistant 4401 class is somewhat better, and the typical temperature limit lies somewhere around 100 to 110 °C. As above, this limit is not absolute, and cases are known in which SCC has occurred in 4401 at temperatures of only 30-40 ºC, particularly in swimming pool environments. Here, whole light equipment has fallen to the ground due to SCC in the cables. SCC and its dependence of the environment and the alloying elements are in detail discussed in SS&C, Chapter 6.4.2 and 6.4.3. Ferritic and duplex stainless steels are much less prone to SCC than the parallel austenites, so if SCC is the primary type of corrosion to be fought, it’s not a bad idea to think in the direction of 4509 or 4521 instead of 4301 or 4404. Intergranular Corrosion (SS&C, Chapter 6.5) Intergranular corrosion (IGC, interkristalline Korrosion, interkrystallinsk korrosion) is a type of corro-sion which is caused by the formation of chromium carbides in the grain boundaries of the steel. Heat-ing the steel to a temperature in the range of 500-850 ºC, carbon is binding the useful chromium (= “sensitization) causing a weakening of the zones adjacent to the grain boundaries. In popular, this corresponds to dissolving the cement in between the bricks of a house. The risk of IGC increases rapidly with the carbon content of the steel, and this risk is the main reason why one should always chose low-carbon steel (i.e. 1.4306, 4307, 4404 or 4435) or titanium stabilized types (4541, 4571), as compared to normal types (4301, 4401). The thicker the steel (= increased heating time), the more important it is to use low-carbon steel. Due to the increased effort of the steel mills in order to remove the carbon, IGC is a rare bird these days.




Fig. 7 Left: IGC in an Asian made 4301 bend containing 0.055 % C. After an un-

lucky heat treatment causing sensitization, the bend was exposed to an acid media “revealing” the problem as IGC.

Right: Micro-section through the same specimen. Note the thickened grain boundaries and the dropped-out grains, both typical for IGC.

Time (SS&C, Chapter 6.6 + Chapter 7) For all types of corrosion, time is a crucial factor. Long-time exposure is always more critical than short-term exposure in a similar environment, and frequently it’s possible to get away with exposing the steel to a, in theory, too corrosive environment, as long as the contact time is kept sufficiently low. This is often seen during disinfection of stainless steel tanks. As long as the disinfection process is kept within a time frame of a few minutes, nothing happens. In contrast, long-term exposure due to left-over disinfectant may give rise to severe corrosion. The contact time is the main reason why the risk of corrosion is the worst when the steel is immersed completely into the media. Hereby, we have the large “bulk electrolyte” and a subsequent risk of ”in-ternal galvanic coupling”, a problem particularly critical to crevice corrosion. Also in the case of pitting corrosion, general corrosion and IGC, conditions are worst under water. In contrast, SCC frequently gets worse above water due to the risk of evaporation and an increase in the chloride concentration. Above water, contact time is essential, and for constructions above water it’s important to minimize the possible contact time. In the case of buildings and tank and pipe equipment alike, it must be en-sured that all water can be drained off as quickly as possible. If not, corrosion may occur, spanning from cosmetically annoying superficial pitting corrosion at low temperature to the destructive SCC at elevated temperature. In general, anything but SCC tends to be of a cosmetic character, however, such attacks may be quite annoying anyway – regardless if the equipment in question is an architect-made post box or the façade of an opera house.




Fig. 8 Kontakttiden er en yderst vigtig faktor for korrosionsforholdene for rustfrit stål.

Under vandlinjen er kontakttiden nærmest uendelig (og uafhængig af overfladefi-nishen), men over vandlinjen er dette ikke tilfældet. Ofte sker korrosion som følge af en for lang kontakt til et korrosivt medie som dette transportsystem (4301), der har været udsat for sprøjt med havvand. Grundige skyl ville have for-hindret korrosionen.

Almost all available corrosion data are based upon long-term exposure. If the contact time can be minimized, chances are that the steel may last far better than the information in the corrosion tables. See SS&C, Chapter 7, and, for a shorter summary, the Damstahl publication dealing with ”Atmos-pheric Corrosion”. All text and photos are © Damstahl a/s and may only be used after a previous written consent from Damstahl. All references are ”Stainless Steel & Corrosion” (SS&C; Claus Qvist Jessen, Damstahl; Oct. 2011) or ”Stain-less Steel for Hygienic Equipment in Food/Pharma” (SS-FP; Claus Qvist Jessen and Erik-Ole Jensen, Dam-stahl; Oct. 2014). Both books are available in Danish, English and Swedish, and SS&C even in German. Both are ordered through www.damstahl.dk.

corrosion of stainless steel - damstahl kort fortalt/ds... · corrosion of stainless steel is...

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