Technical Presentations at the July 2003 Meeting

2.1  ‘Microbial Induced Corrosion on Stainless Steels in the Final Stages of Wastewater Treatment’, Anna Iversen (AvestaPolarit) 

Exposure trials have been completed to study Micro biologically induced corrosion (MIC) on stainless steels in waste water treatment plants. The objectives of the work were to explain one or two unexpected failures and to provide a stronger basis for materials selection in this application. 

Samples were exposed in the final settling tanks of 6 waste water treatment plants. Both aerobic and anaerobic systems were considered. 

It was found that MIC was associated with ennoblement of the stainless steel and that this ennoblement was dependent on the oxygen and nutrient contents of the water. 4301 (304) performed most poorly of the alloys considered; 4436 (316) was also attached by MIC but not to the same extent, both alloys at chloride levels where survival would have been anticipated in aerobic systems. 2205 duplex stainless steel was not attached. 

A risk assessment diagram has been developed and it was presented.

 

2.2  ‘How does current EU Legislation effect the Performance of Future Fastener Coating Processes?’,  David G James (Surface Engineering Specialists Ltd)

Of the several different ‘Traditional’ Sacrificial Fastener Coating Systems, in general terms, the electroplating or the dip spin process has become the best value for choice processing route for protective coatings for fasteners.  Each process however relies upon the presence of other materials to enhance corrosion protection. 

Electroplated fastener coating systems have developed over the last 50 years or so to provide reliable, cost effective finishes that satisfy today’s demands for longer life, attractive appearance and assured in place performance. 

What we are starting to experience now is the effect of the implementation of the EU policy on environmental protection known as “The Sixth Environment Action Programme of the European Community 2001-2010”.  This legislation will regulate:

  • health and safety in the work place

  • the materials we use

  • the way we pollute the environment

 The metal finishing industry and its customers face legislation that affects the profitability and sustainability of their business, and the choice of material used in the manufacturing process. 

It will be necessary to replace hex chrome with trivalent passivates.  All major car companies now have hex chrome free specifications in place, and applications for components plated with the traditional hexavalent passivated zinc will become the exception rather than the norm. 

The new Cr+6 replacement processes are characterised by higher manufacturing costs than hexavalent processes, and higher operating costs than hexavalent processes.   Those affected will initially be the automotive and electrical/electronic manufacturing companies, then their First Tier Suppliers, and their suppliers, but eventually all supply chain metal finishing suppliers.

 

4.1   ‘Corrosion in 40 Minutes - Evaluating the Susceptibility of Welds’, Bill Nimmo (NPL)

Localised corrosion in welded structures can be caused by poor selection of materials, processes, or the failure to adhere to specified procedures.  It is necessary therefore to be able to rapidly identify potential problems. Recent research at NPL, in collaboration with industrial partners, indicated that the galvanostatic test reproduced the same behaviour as long term immersion tests on three sample welds in carbon steel manufactured to give specific types of problem.  A draft version of the method used is available at http://www.npl.co.uk/npl/cmmt/aqueous/oil_and_gas/downloads/matc_a_117_guide_to_use_of_the_galvanostatic_method.pdf.

The scanning vibrating probe method was also found to be a useful tool and provided maps of  corrosion activity across the section of the welds.  Hardness patterns derived from hardness profiling and hardness scanning were found to be unreliable predictors of corrosion susceptibility.  The full version of this paper is available at http://www.npl.co.uk/npl/cmmt/ncs/welds/carbon_steel/

A review of test methods carried out at the start of the project is available at http://www.npl.co.uk/npl/cmmt/aqueous/oil_and_gas/downloads/cmmt_a_130_corrosion_testing_of_welds.pdf.

 A conference on the corrosion of welds is being organised; for more information go to http://www.npl.co.uk/welds_conference/ .

 

4.2  ‘The Behaviour and Application of Stainless Steels in Water Treatment Plants & Distribution Systems’, Carol Powell (NiDI)

 Water intake at the treatment plant can come from a number of different sources. Surface waters come from rivers, lakes and reservoirs, which may have a wide range of chemistries with high mineral and metal contents, chloride levels and particulates.  Ground waters taken from underground springs, water tables and wells are low in oxygen or fully de-aerated with varying amounts of hydrogen sulphides and sulphate reducing bacteria. 

The incoming water streams are usually screened and then treated with oxidising chemicals such as chlorine and potassium permanganate, to precipitate iron and manganese.  Filters remove these precipitates, floc and particulate matter.  Inside the treatment plant, additional chemical treatments using ferrous sulphate, alum, caustic soda and other reagents assist in neutralising water streams and the precipitation of metallic ions and particulate matter.

The treated waters will often undergo a final filtration process, using stone/gravel filter beds or activated carbon, followed by a final disinfection by an oxidant such as chlorine before distribution.

 Distribution systems essentially cover the delivery of clean, treated water from the water treatment plant to the point of use. The water should remain clear, colourless and odourless while within this system.

The use of stainless steel in water treatment and distribution has increased in the UK and many countries internationally with  304L and 316L type grades of stainless steel being the standard materials of construction, and duplex and super austenitic alloys considered for more arduous service.  Stainless steel provides a material with extremely low corrosion rates in the handling of a very wide range of waters.   Unlike the traditional construction materials, water chemistry control is not required to prevent corrosion attack, although bactericide treatments will still be necessary as for all potable water streams.

 With excellent corrosion-erosion characteristics in high flows of water, stainless steel can easily handle changes of cross-section, high aeration, pumping turbulence, and high velocities. Stainless steel systems require no corrosion allowance or coatings and can be designed using thin walls. Also, as higher flow velocities can be accommodated, smaller cross-sectional piping sizes can be used for the same mass flow rate that would be permissible with conventional materials.

 Table 1 provides a list of applications where stainless steels have been successfully used within water treatment plants. 

Well linings and filters           Chemical treatment lines                          Pumps

Well head components          Reservoir linings                                     Valves

Slide gates                            Platforms                                                Tanks  

Weirs                                    Bridges                                                  Ladders

Overflows                              Electrical fitting covers                            Railings

Screens                                  Manhole covers

Scrapers                                 Granulated Activated Carbon Towers

Fasteners                                Ozone Generators

Piping                                      Ozonation piping

Ventilation stacks                     Central control gravity filters

Sampling and sensor lines         Pressure doors and frames

 Stainless steels do not suffer uniform corrosion when exposed to water environments.  However, they can be susceptible to localised corrosion under certain sets of circumstances which designers and end users need to be aware of and take actions to avoid.  Such attack, if it occurs in water environments, is usually localised in creviced areas. Attention to correct material selection, detail during welding, ensuring complete through-wall weld penetration and the use of welding and inspection procedures can help ensure good service. 

 Chlorides in waters are not the only factor that influence the localized corrosion behaviour and alloy selection of stainless steels but they a measurable first indicator. Based on laboratory tests and service experience over many years, the guidelines given in Table 2 may be used for waters at ambient temperatures with pH levels above 6, typical of most waters. They suggest chloride levels below which crevice corrosion has been found to be rare.

 Table 2. Chloride Level Guidelines for Waters

Condition

Alloys

<200ppm chlorides

304, 304L, 316, 316L,

200-1000ppm chlorides

316, 316L, 2205

 

1000-3,600ppm chlorides

Duplex alloy 2205

6%Mo superaustenitic and superduplex grades

>3,600 ppm chloride

6%Mo superaustenitic and superduplex grades

Sea water

6%Mo superaustenitic and superduplex grades

Where severe conditions might occur e.g. very tight crevices, lower pH, higher temperatures, low flow and other conditions where there is a risk that higher chlorides might concentrate locally, or just for conservatism, upper chloride levels in the region of 50 ppm for 304L and 250 ppm for 316L can be preferred. 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 values in the Table 2 can be relaxed.

 Hydrostatic testing of water lines and vessels represents a very important approach in checking the integrity of systems after construction. However, it is very important to drain and dry stainless steel systems promptly after testing, if the equipment is not going into service directly. Alternatively, maintaining regular flushing of the system on a daily basis should limit potential problems. This is particularly important in handing raw waters, where bacteria and water stream sediments can settle out and initiate corrosion attack in the area of welds.  Removal of heat tint scale restores base metal corrosion resistance and greatly improves resistance to microbiologically influenced corrosion (MIC). Potable waters should be used for testing rather than raw waters.

 Bacterial control and management is often achieved by chlorine dosing. Type 316L stainless steel performs well and the molybdenum additions in this alloy provide greater pitting and crevice corrosion resistance than its Type 304L  counterpart. Data to evaluate acceptable free chlorine levels is limited but that available for raw waters suggest up to 2ppm for type 304L and 5ppm for type 316L.  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. 25 ppm chlorine for 24 hours. It is important however that such levels are well flushed through the system immediately after treatment.

Care must be taken when adding chlorine and chlorine compounds to various water 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.

 In areas where wet chlorine vapours collect and concentrate, condensates can begin to stain and pit stainless steel. In treatment plants, good venting or regular washing down is recommended in pipe galleries and other areas where chlorine gases can collect. Where this is not possible, a higher grade of stainless may be required.

 Ozonation of potable waters 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 as chlorine, and is environmentally friendly.   Type 316L stainless steel is a standard material used in ozone generation and for the handling of the ozonated water streams.

In general, the full benefits of stainless steels can be achieved in the water industry by attention to the following:-

·        For optimum performance, consideration needs to be given to good fabrication procedures, principally, making full through-wall welds, removal or minimising heat tint, and cleanliness.

·        Systems which are not put into service directly after hydrotesting should be drained and dried in order to avoid potential problems especially if raw waters are used. If this is not possible, the waters should be circulated regularly.

·        Flowing conditions should be maintained where possible.

·        During operation, oxidising chemicals injected directly at or along the walls of stainless steel and excessive dosing should be avoided.

·        Venting or regularly wash down of areas where chlorine vapours can collect.

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