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The different uses of stainless steel |
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There are more than one hundred different kinds of stainless steel to choose from. In the employment of stainless steel one distinguishes basically between unprotected weathering and contact with water.
In the open atmosphere, mainly the aggressiveness of the air but also the humidity of the air and the temperature play decisive roles (see table).
| material | ferritic steel | austenitic steel | | application place | 1.4301 | 1.4541 | 1.4571 | at the country side or in
usual atmosphere of towns | partly suitable | suitable | suitable | suitable | heavily polluted
industrial atmosphere | not suitable | partly suitable | partly suitable | suitable | atmosphere at oceans
with high salt contend | not suitable | not suitable | not suitable | suitable | Table: Suitability dependent on material under atmospherical conditions
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Direct contact with water |
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When there is direct contact with water, the chemical composition and the temperature have to be observed. Due to the danger of localized selective corrosion the percentaget of halogen ions, especially chlorides, play an important role. But also other effects, such as the direction of the heat transition or the pH-value have to be considered. Values for orientation are shown in the table below. With aggressive waters it may be necessary to do a corrosion study in the lababor.
| Gr. | Einsatz | Cl- in case of ambient temp. | Cl- in case of 70° | example | | mg/l | mg/l | | 1 | drinking and industrial water | 100 | 50 | 1.4301, 1.4541 | | 2 | drinking and industrial water | 400 | 200 | 1.4571, 1.4401 | | 3 | cooling, service and mineral water | 1,000 | 500 | 1.4435, 1.4436 | | 4 | sea, brackish water brine | 20,000 | 5,000 | 1.4439, 1.4539 | | >5 | sea, chemical water | 25,000 | 10,000 | 1.4566, 1.4562 | Table: Chlorine and chloride are very important concerning the so-called localised corrosion, which depends on the existance of ozone, neutral salt and also on the temperature. It is not disturbing if the steel has a short-term contact with chlorine, but if there is a constant strain it is necessary to check the steel.
Picture 1: Stainless steel pipelines
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Picture 2: pipes, which have rusted after short time in stock
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Corrosion behaviour of non-rusting steels in different media In the following sections, the behaviour of corrosion-proof steels in different corrosive media that are divided into typical groups will be discussed.
Destilled water
Common 18/8-Chromium-Nickel-steel practically does not get attacked by destilled water. For example, samples of the steel 304 1400 h were exposed to destilled water from a condensator. The loss in weight corresponded to a removal of only 5 m/year. In short-term trials with hot or cold destilled water, one normally does not find any weight losses with 18/8-steel samples.
Tap water
Long-term studies at different locations with 60° C warm tap water showed that 18/8-Chromium-Nickel-steel is extemely resistant in this medium. The table below shows several corrosion values that were generated with a water that was particularly corrosive for many materials.
| study location | duration | corrosion speed | depth in hole | | in d | in g/(m² d) | in µm/a | in µm | | Hollis, L.l. | 1555 | 0.00006 | < 3 | 25 | | Pittsburgh, Pa. | 466 | 0.002 | < 3 | 25 | | Kingston, N.Y. | 1488 | 0.0001 | < 3 | 75 | | Camden, N.Y. | 204 | 0.004 | < 3 | 0 | Table: Corrosion behaviour of steel 304
For use in tap water, usually the common 18/8-Chromium-Nickel-steel is sufficient while the Molybdenum-containing type 316 is more suitable for water with extraordinary elevated chloride content.
Corrosion-free steel devices used for corrosive water should not have any unsealed crevisses in order to avoid crevice corrosion.
River water
In most cases, corrosion-free Chromium-Nickel-steels are totally resistant to river water even if the water is so acidic that other materials are affected strongly. The following table shows the results of corrosion studies that used river water. Localized selective corrosion did not occur in these studies. | study location (river) | duration | corrosion speed | | in months | in g/(m² d) | in µm/a | | Allegheny | 330 | 0.0006 | < 3 | | Monogahela | 338 | 0.0012 1) | < 3 | | Monogahela | 128 2) | 0.003 3) | < 3 | | Potomac (Hagerstown, Md.) | 394 2) | 0 3) | < 3 | | Mississippi | 1095 | 0 | < 3 | | Savannah | 148 | 0.001 | < 3 | | Colorado | 300 | 0.002 | < 3 | | Delaware (Wilmington, Del.) | 70 | 0.003 | < 3 | | Hudson (Poughkeepsie) | 60 | 0.014 | < 3 | Table: Corrosion behaviour of steel 302 in river water
1) As comparison: for un-alloyed steel, the corrosion velocity for this trial was 700 µm/year
2) This trial was done with hot water in a condenser
3) As comparison: with CuZn28Sn (SoMs71) the corrosion velocity for this trial was 330 µm/year
Corrosion-resistant steels are also extremely resistant under conditions of erosion and cavitation as they can occur on the blades of water turbines or on ship propellers. The Table below depicts the results from cavitation trials. In these trials, the materials-to-be-tested were welded onto turbine blades in areas which were either considerably damaged or where an increased resistance against cavitation was to be achieved. | AlSI-Type 304, 1.4301 | rolled | 153 | 13.9 | | AlSI-Type 304, 1.4301 | casted | 135 | 11.8 | | AlSI-Type 302, 1.4300 | rolled | 182 | 3.7 | | AlSI-Type 309, 1.4828 | rolled | 139 | 8.6 | | unalloyed steel (0,30% C) | rolled | 145 | 135.0 | | unalloyed steel (0,33% C) | casted | 159 | 62.4 | | pig iron (3,18% C) | casted | 171 | 636.0 | Table:Results from cavitation trials in fresh water
Boiler water
In general, boiler waters have compositions that do not affect non-alloyed steels greatly.
The austenitic Chromium-Nickel-steels are resistant to boiler waters even under conditions of high pressure and temperature. For instance, the corrosion velocity for samples of steel 316 that were subjected for 64 days in a feed water heater to a water with pH 8 - 8.5 at a pressure of 79 at and a temperature of 200° C was less than 3 m/year; localized selective corrosion could not be observed either. Similar results were found for the steels 304 and 316 in a study lasting 232 days with a temperature of 300° C and under significantly higher pressure.
Extremely pure cooling water used for water-cooled nuclear reactors differs from the common boiler water. It´s pH is adjusted to a value of about 10 using Lithium- or Ammoniumhydrate and hydrogen may have been added to the water. Through intensive studies at temperatures up to 320° C it was found that austenitic Chromium-Nickel-steel are extraordinary resistant to this medium. Many reactor components that come into contact with such a water, were made out of austenitic steels.
Neutral and alkaline salts (with the exception of halogen salts)
Austenitic Chromium-Nickel-steels are totally resistant to neutral and alkaline salts even when the salts are strongly oxydizing.
Acidic salts (with the exception of halogen salts)
The austenitic Chromium-Nickel-steels behave towards acidic salts in the same way as they would react towards acids that would form from these salts due to hydrolysis. The acidity and as a result the corrosive effect of a salt solution is naturally lower compared to that of a pure acid.
The normal, corrosion-resistant steels are resistant to acidic salts within a wide range of concentrations and temperatures. If the acid strength and the temperature are high it would be better to use Molybdenum-containing types of steel.
Halogen salts
Halogen ions can pass through the passive layer of Chromium-Nickel-steels and can cause localized selective corrosion. This can happen especially if the salt solution is acidic and oxydizing. For a salt solution that is acidic and reductive, the entire surface may be strongly attacked. The following information that refers to a solution of sodium chloride can also be applied for general cases and for different halide solutions that are neutral or slightly alkaline.
It was observed that the corrosion due to localized selective corrosion apparently increases dramatically above a certain critical temperature. This critical temperature is markedly higher for Molybdenum-containing kinds of steel, e.g. the type 316, than for Molybdenum-free kinds. BRENNERT found that the critical temperature of a 3 n sodium chloride solution for the steel 304 is approximately 55° C , for the steel 316, however, the critical temperature is greater than 70° C. According to UHLIG and MORRILL, the corrosion of steel 304 strongly increases above temperatures of 6°0 C and reaches its maximum at 91° C. For even higher temperatures, corrosion due to localized selective corrosion decreases again due to lower oxygen solubility. Furthermore it was observed that the corrosion increases significantly up to a sodium chloride concentration of 4 % while above this concentration it increases only slightly.
According to UHLIG, localized selective corrosion occurs in a 90° C, 4% sodium chloride solution with a pH greater than 2.8; for lower pH values, however, a fast-spreading surface attack occurs. The deepest holes were created for pH values of 6...7. If the alkalinity increases, the attack by localized selective corrosion decreases and is reduced to a minimal level at a pH of 12.
For further information on this topic please refer to the print medium "Corrosion resistance of austenitic Chromium-Nickel-steels towards Sea Water", International Nickel Germany GmbH, Düsseldorf, 1967. Please notice that the material designations used here are refering to US standards. The respective German designations are:
- 302 - 1.4300
- 304 - 1.4301
- 309 - 1.4828
- 316 - 1.4571
- 321 - 1.4541
- 3t6 L - 1.4435
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HUBER - WHERE QUALITY AND SERVICE SPRING TO MIND
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Bumpers Farm, Chippenham Wiltshire SN14 6NQ
Phone +44 1249 765000, Fax +44 1249 449076, E-mail: rotamat@huber.co.uk |
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