Guidelines for Alloy Selection for Waters and Waste Water Service

The most important considerations to achieve optimum corrosion performance are to:

  • choose the correct grade for the chloride content of the water;
  • avoid crevices when possible by good design;
  • follow good fabrication practices, particularly removing weld heat tint;
  • drain promptly after hydrotesting.

Waters

Stainless steels do not suffer uniform corrosion when exposed to water environments. On the other hand, they can be susceptible to localised corrosion under certain circumstances which designers and end users need to recognise and avoid. Such attack, if it occurs in water environments, is usually localised as pits or in creviced areas. Design and good fabrication can minimise such corrosion sites but this needs to be combined with correct alloy selection.

Pitting and crevice corrosion requires the presence of chlorides and, for a given chloride level, the more highly alloyed stainless steels are more resistant. In general, the higher the chromium, molybdenum and nitrogen contents of the steel, the better the corrosion resistance. While there are other factors that have an effect on corrosion rate in waters, chloride content is a major factor for selection of an appropriate grade and is easily measurable. As crevice corrosion tends to occur at lower chloride levels and temperatures than pitting, it is normally the parameter used to guide selection. The guidelines in Table 4 are based on laboratory tests and service experience over many years.

Table 4. Suitability of Stainless Steels in Waters

Chloride Level ppm

Stainless Steel Grade

<200ppm

304L, 316L

200-1000ppm

316L,duplex alloy 2205

1000-3,600 ppm

Duplex alloy 2205

6%Mo super-austenitic, super-duplex

>3,600ppm

6%Mo super-austenitic, super-duplex

15,000 – 26,000 ppm(seawater)

6%Mo super-austenitic, super-duplex

For the chloride levels given in  Table 4, crevice corrosion is rare at pH levels above 6 and ambient temperatures, typical of most water industry environments. However, where conditions are severe, e.g. very tight crevices, lower pH, higher temperatures, low flow rates and other situations where there is a risk of local concentration of higher chloride levels - or just on grounds of conservatism - 304L(1.4307) can be selected for chloride levels in the region of 50ppm and 316L(1.4404) for chloride levels up to 250ppm. Alternatively if the stainless steel is cathodically protected, the waters are de-aerated, or there is only transient exposure to these chloride levels then the requirements in Table 4 can be relaxed.

Machining grades of stainless steel contain high levels of non-metallic stringers which significantly lower their resistance to pitting in waters. Therefore, free-cutting high sulphur or selenium bearing grades such as alloy 303 and 303Se should not be used.

Chemical Additions

Care must be taken when adding chlorine compounds to various process streams. Serious consideration needs to be given to ensuring that chlorine and aggressive chemicals, such as ferric chloride (added for flocculation purposes), are added centrally into the stream for good dispersion. Concentrated forms of these chemicals directed at or down the side of stainless steel piping or equipment can result in localised attack.

Bacterial control and management is often achieved by chlorine dosing. Type 316L(1.4404)  stainless steel performs well and the molybdenum additions in this alloy provide greater pitting and crevice corrosion resistance than its Type 304L(1.4307) counterpart. Data to evaluate acceptable free chlorine levels is limited but that available for raw waters suggest up to 2ppm for type 304L(1.4307) and 5ppm for type 316L(1.4404).  However, stainless steel can tolerate considerably higher levels of chlorine for short periods of time, as would be the case during disinfection treatments e.g. AWWA Standards C651/652 where 25-50 ppm chlorine are held for 24-48 hours. It is important however that such levels are well flushed through the system immediately after treatment.

Useful reference: Effect of Chlorine on Common Materials in Fresh Water. NI Publication 14049

Ozonation has increased in popularity.  This is a powerful oxidant with limited retention life.  It does not create ions or compounds which are as aggressive to stainless steel. However, a good filter is preferred to remove sediment from the cooling water before it enters the generator to avoid deposit build up on the tubes.   Type 316L(1.4404)   stainless steel is a standard material used in ozone generation and for the handling of the ozonated water streams.

Flow and Stagnation

Stainless steels do not suffer from erosion corrosion and can operate at high flow rates, up to around 40m/s without breakdown of the surface film. Flow rates of greater than 1m/s are preferred in raw waters and greater than 0.6m/s in cleaner, treated waters to avoid deposit build up.

Prompt removal of stagnant water after hydrotesting requires particular attention. It is very important to drain and dry stainless steel systems after hydrotesting, if the equipment is not going into service directly. Alternatively, if this is not possible, maintaining regular flushing or water recirculation of the system is good practice. Potable waters, steam condensates (where available) or filtered waters should be used for hydrotesting rather than raw waters.

The above practices avoid long term stagnant conditions that occasionally can produce colonisation of certain unsuitable types of bacteria as biomounds and tubercles which can lead to microbiologically influenced corrosion (MIC). The bacteria causing MIC are more likely to colonise in the area of welds that have not been cleaned of heat tint. Therefore, good fabrication procedures which remove or avoid heat tint also greatly improve resistance.

Useful Reference: Microbiologically Influenced Corrosion of Stainless Steel by Water used for Cooling and Hydrostatic Testing, NI Publication 10085

Plant Atmospheres

Water TreatmentWhen fabricated and finished to suitable standards, the type 304L(1.4307) and 316L(1.4404) can retain their bright appearance in atmospheric exposure for many years particularly when any surface deposits which build up are removed by a periodic wash down. In marine (within 5-15 miles of the coast depending on wind and temperature) and chloride bearing or industrial polluted atmospheres, the 316L(1.4404) is preferred where maximum life and good appearance are required.

(For more information on atmospheric exposure, particularly where an aesthetically pleasing appearance is required, see  Preventing Coastal Corrosion (Tea Staining) ASSDA Publication

In enclosed plant atmospheres, where wet chlorine vapours collect and concentrate, acid condensates can begin to stain and pit stainless steel. Good venting or regular washing down is recommended in pipe galleries and other areas where chlorine gases can collect. However, where this is not possible, a higher grade of stainless such as alloy 2205(1.4462) or super stainless alloys may be required.

Galvanic Behaviour

It is often necessary to use a number of different alloys to construct a treatment plant or processing system and the galvanic compatibility of these materials must be considered. Galvanic corrosion can occur when 2 different alloys are in contact in a common electrolyte  (e.g. rain, condensation, fresh and treated waters and waste water) forming a galvanic corrosion cell. If current flows between the two, the less noble alloy (the anode) corrodes at a faster rate than it otherwise would if the alloys were not in contact.

Prediction of corrosion rates can be complicated by area ratios of the metals, temperature, surface films and the electrical conductivity of the electrolyte and are not always easy to accurately assess. Galvanic series are available to indicate which alloy is the least noble in a metal couple but these are usually based on sea water as the electrolyte. Less information is available about other waters but Galvanic Corrosion-A Practical Guide for Engineers by Roger Francis published by NACE Press goes some way to assembling existing information.

A typical ranking for fresh water is:

ANODIC—LEAST NOBLE

  • Magnesium
  • Zinc
  • Aluminium
  • Carbon steel and cast iron
  • Copper Alloys
  • Stainless steels
  • Graphite

CATHODIC—MOST NOBLE

The greater the potential difference or, put more simply the vertical separation, between two metals in the series, the greater the driving force for corrosion. Stainless steels are noble alloys in this ranking and they are the protected component of the combination.

When the stainless steel is active such that the passive film is damaged or removed by localised corrosion attack, its position in the series can change to between copper alloys and carbon steel and thus become less noble.

In practice galvanic corrosion is particularly relevant when considering joining stainless steel and carbon or low alloy steels. The risk of deep attack is greater if the stainless steel area is large compared to the steel; for example, galvanised or steel fasteners in a stainless steel flange.

Methods of avoiding galvanic corrosion are:

 

  • Design to ensure the more noble area is small in comparison with the less noble area
  • Insulate the Joint e.g. insulating gaskets, sleeves and washers, paint and tapes.
  • Use of Isolation spools
  • Cathodic protection
  • Coating the joint region ensuring adequate coverage either side. If this is difficult coat only the more noble metal. If the less noble metal alone is protected, then attack at any coating defects will be severe.
  • Galvanic corrosion is also a reason why it is important to select a weld consumable that is at least as noble as the parent metal.

In fresh waters as opposed to sea water, copper base alloys are compatible with stainless steel unless extreme stainless steel to copper area ratios exist for example copper alloy valves can be used in stainless steel piping. However, steel, zinc and aluminium are significantly less noble that stainless steel and generally should be insulated from stainless steel.

Useful Information: Stainless Steel in Waters: Galvanic Corrosion and its Prevention, A.E. Bauer

Waste Water Processing

Hydrogen sulphide gas can contribute to the general corrosion that occurs on copper alloys, aluminium alloys and hot dipped galvanized steel, painted/unpainted steel in wastewater treatment plants. In contrast, general corrosion rates of 304L(1.4307) and 316L(1.4404) stainless steels in the atmosphere and in closed systems (e.g. pipework), where moist hydrogen sulphide is present are negligible at near ambient temperatures. However in closed systems there may be a propensity for localised corrosion attack (pitting and crevice corrosion) to occur in 304L(1.4307) and 316L(1.4404) stainless steels if moist hydrogen sulphide and chlorides are present together at elevated temperatures. The acidity of wastewaters may also be raised so that they become more corrosive if condensates containing dissolved sulphur dioxide are generated, forming sulphurous acid.  These more corrosive environments may require higher molybdenum austenitic stainless steels (e.g. alloy 904L/1.4539) or duplex stainless steels (e.g. alloy 2205/1.4462) to be considered as materials of construction.

Useful Guideline Reference:
Applications for Stainless Steel in the Water Industry, IGN 4-25-02, WRc. UK (1999)
Stainless Steel in Municipal Waste Water Treatment Plants, NI Publication 10076

Soils

Soil corrosivity towards stainless steels depends on many factors the more important of which are  soil resistivity, pH, chloride content and soil drainage. Resistivity provides a guideline  to the soil’s water retentiveness e.g clay; sand; loam. and the higher the resistivity, the better is the drainage. Chlorides  are affected by location; the more aggressive being coastal sites or near roads salted for deicing.  Selection of bedding can help with drainage away from piping, especially in aggressive acidic and/or high chloride soils.

Coating protection, cathodic protection or both is often suggested  for type 300 stainless steels in conditions where the resistivity is less than 2,000 ohm.cm, pH is less than 4.5 and drainage is poor. Between resistivities of 2000-5000 ohm.cm, stainless steels may require protection or consideration given to higher alloys if the chloride level is high or soil acidity is of concern. In the EuroInox paper, Stainless steels in Soils and Concrete, recommendations are given in more precise terms and provides guidance for material selection vs chloride levels in the absence of coatings and or cathodic protection. A summary is given in table 5.

Table 5. Stainless Steel Selection Criteria According to Soil Conditions

Stainless Steel

Soil Conditions

304L

 

316L

Chloride <500ppm

 

Resistivity>1000 ohm.cm

pH.>4.5

316L Chloride<1,500ppm

 

Resistivity>1,000 ohm.cm

pH.>4.5

Super duplex

 

Super austenitic

Chloride<6,000ppm

 

Resistivity>500 ohm.cm

pH>4.5