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CHLORIDES AND STAINLESS STEEL

By Michele Hawkins,2014-07-14 14:20
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CHLORIDES AND STAINLESS STEELand

     ENMAX LTD

    PERMITTED CHLORIDE LEVELS FOR STAINLESS STEEL IN WATER.

     ndVersion 1, 2 April 2011

Introduction.

    What is a safe level for the chloride content of water in contact with stainless steel? There is no clear answer on this because it depends on so many factors grade

    of steel, surface condition, presence of crevices, temperature, presence of other species in the water and so on. This article is intended to review the limits set by various workers for nominally clean or potable water and to draw some conclusions about the amount of chloride that can be tolerated by the common grades of stainless steel the austenitic grades 304L and 316L and the duplex

    grade 2205.

Failure modes.

    General corrosion of stainless steels in water is generally not a problem but, in the presence of chloride ions, localised corrosion mechanisms such as Stress Corrosion Cracking (SCC), Crevice Corrosion and Pitting can cause serious failures.

     SCC: Austenitic stainless steels are particularly prone to SCC at 12temperatures above ambient. NACE and NORSOK recommend that austenitic ?stainless steels should not be used above 60C in the presence of chlorides a ?3,10,4more conservative value of 50C is recommended by others. The higher

    alloyed steels such as 2205 are less susceptible to SCC, crevice and pitting ?corrosion: NACE and NORSOK state at least 100C can be tolerated.

     Crevice Corrosion: As the name implies, crevice corrosion occurs at

    crevices which need only be of the order of microns wide to cause problems

    surface laps in the steel surface, flanged joints, seals, deposits, rivets, bolts, etc. are all potential sites for crevice corrosion. Crevice corrosion is usually easier to initiate than pitting in essence the crevice is a pre-formed pit.

     Pitting Corrosion: Pitting corrosion occurs at points of localised

    breakdown of the passive film that protects the steel from corrosion often at

    surface breaking inclusions where the passive film has been prevented from

     1 NACE MR0175/ISO15156-3 2003 Petroleum and Natural Gas Industries. Materials for use in HS-2

    containing environments in oil and gas production. 2 NORSOK Standard M-001 Rev 4 2004 Materials Selection. 3 Hargreave, R.E. Behaviour of 300 series Stainless Steel Heat Exchangers in Cooling Water Service. Paper no 04080 Corrosion 2004. 4 Atlas Steels Australia (at www.AZOM.com)

    forming. If crevices are absent, then the conditions for pitting corrosion need to be defined.

    For the permitted chloride levels discussed in this paper, it is assumed that crevices are present.

Clean/Potable Water.

One of the most commonly cited reference for chloride limits in water is that from 5the Nickel Development Institute (NiDi) which suggests that for 304L in natural,

    raw and potable waters at a pH of 6.5-8 and a chloride level of 200ppm, crevice corrosion is rare. No temperature is given but it has been assumed to be approx. ?15C. Where conditions are less favourable, a more conservative value for chloride content is 50ppm maximum. The corresponding figures for 316L are 1000ppm and 67250ppm respectively. These figures are cited by Cutler, SSINA, Water Industry 891011Information and Guidance Note, BSSA, Drinking Water Inspectorate, ASSDA

    and others. They are said to be based on laboratory trials supported by service experience and so are a good starting point when considering chloride limits for a particular application.

Effect of aeration.

    What role does aeration play in the corrosion of austenitic stainless steel? 5According to NiDi deaeration is beneficial in reducing the tendency for crevice corrosion to occur. The reason is that the cathode reaction which controls the corrosion within the crevice is oxygen reduction and is suppressed when little or no oxygen is present.

Effect of temperature.

     12Data presented by Outokumpuindicate that in oxygen saturated water, increasing

    the temperature increases the risk of crevice corrosion. For example, 304L has a ?chloride limit of approx. 190ppm at 15C for crevice corrosion (similar to the NiDi ?value) but this reduces to approx. 22ppm at 60C; the corresponding figures for

    316L are approx. 465ppm (about half the NiDi figure) and 125ppm. For 2205 the ??5figures are 3800ppm (15C) and 515ppm (60C). In contrast, NiDi suggest that

    higher temperatures reduce the tendency for crevice corrosion because the solubility of oxygen in the bulk water is lower. As stated in the previous paragraph, this occurs because it has the effect of reducing the cathodic reaction thus 13reducing corrosion within the crevice. In another paper, NiDi state that in fully de-

     5 Nickel Stainless Steels for Marine Environments. Natural Waters and Brines. A Nickel Development Institute Reference Book. Series No. 11 003, 1983. 6 Cutler, P. stainless Steels and Drinking Water around the World. Nickel Development Institute. 7 Specialty Steel Industry of North America. Stainless Steel in Water Handling and Delivery Systems. 8 Water Industry Information and Guidance Note IGN 4-25-02 Jan 19999. Applications for Stainless Steel in the Water Industry. 9 British Stainless Steel Association. 10 Drinking Water Inspectorate. Operational Guidelines and Code of Practice for Stainless Steels in Drinking Water. 11 Australian Stainless Steel Development Association. 12 Outokumpu datasheet for Standard Cr-Ni-Mo stainless steels. March 2006. 13 Avery, R.E. et al Stainless steel for potable water plants Guidelines.

    aerated waters much higher levels of chloride can be tolerated by the austenitic stainless steels even sea water with approx. 18.000ppm chlorides.

Summary.

    As can be seen, there are differences in the recommendations from the various sources reviewed. The reader is invited to draw his/her own conclusions on which recommendations are applicable for a given situation. The only way to ensure no risk from chlorides in industrial processes is to ensure that none are present by using demineralised water, a strategy adopted, for example, by the nuclear industry. For less critical applications, the chloride limits shown in Tables 1 and 2 are suggested by the writer. Table 1 has been compiled from the NiDi 5?recommendations for temperatures of the order of 15C; for higher temperatures

    the chloride limits have been reduced using the chloride/temperature relationship 12indicated by Outokumpu. The conservative values shown in Table 2 are

    approximately 25% of the non-conservative values.

Disclaimer.

    The values for permitted chloride levels in water shown in Tables 1 and 2 should be considered as a guide only. The reader should always refer to the original data sources quoted in this paper and derive acceptable chloride levels based on his/her particular situation.

T.Roberts,

    Enmax Ltd.

    TABLE 1: NON-CONSERVATIVE VALUES

    Material Chloride Limit, ppm Comment Temperature, ;C

    304L 15 200 Do not use above ?60C in the 60 20

    presence of 316L 15 500

    chlorides because 60 120

    of risk of stress

    corrosion cracking. 2205 15 3800 Do not use above

    100 deg C in the 60 520

    presence of 100 100

    chlorides because

    of risk of stress

    corrosion cracking.

    TABLE 2: CONSERVATIVE VALUES

    Material Chloride Limit, ppm Comment Temperature, ;C

    304L 15 50 Do not use above ?50C in the 50 5

    presence of 316L 15 125

    chlorides because 50 30

    of risk of stress

    corrosion cracking. 2205 15 1000 Do not use above

    100 deg C in the 50 130

    presence of 100 25

    chlorides because

    of risk of stress

    corrosion cracking.

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