PROTECTION AGAINST CORROSION
BEAUDREY has worked hard to resolve this problem and acquired much experience.
Corrosion protection of a screening plant is a difficult subject because conditions vary from site to site and while the basics of corrosions are well known, the ways they do or do not apply are somewhat unpredictable as far as screens are concerned.
Screening plant equipment deals with polluted water for which there are five types of pollutants:
Shell and algae growth
De-aerated stagnant water
All five types have an influence on corrosion. To be added to this are the effects of water velocity and wash-water spray. Most parts of the screening equipment are subject to cycles of emersion and immersion.
Adverse effects of solids are mainly those of shocks and abrasion which locally damage the protective coating of the steel. This creates starting points for corrosion.
Dissolved chemicals attack paints and metals alike.
Bacteria attack the metal either by thriving on metal oxide or by producing chemicals that help corrosion. Some bacteria eat up steel in stagnant water.
Biological growth such as shell-fish help corrosion by creating stagnation zones and differential aeration of the surface.
FACTORS THAT INCREASE CORROSIVE ACTIONS
Salt in water is a major corrosion-increasing factor. Variations in salinity seem to increase corrosion.
H2S (Hydrogen Sulfide)
This can be generated either by vegetal decay (algae) or by man-made pollution. It is especially active on copper alloys and steel. Stations which are subject to stagnation, vegetal pollution or sludge deposits are prone to H2S. Natural surges of hydrogen sulfide are sometimes seasonal.
Heat and Light
Warm water increases chemical activities like corrosion. Warm air and sunlight spoil many types of paint. What is stated here applies to temperatures below 50°C.
Water stagnation induces de-aeration of the water, helps in breaking up self-protecting stainless-steel oxide coating and creates other types of corrosion (bacteriological and hydrogen sulfide).
Internal stresses tend to alter metal potentials and thus change corrosion-resistance capacities. Welds, heavily-stressed zones and high-stress machining can be vulnerable zones for corrosion.
Stray currents can also create unexpected corrosion. They are not always easy to stop. Badly-set impressed current cathodic protection can increase corrosion.
Electro-voltaic currents between different materials can create violent corrosion. Metals with different potentials sometimes do not give the expected trouble. BEAUDREY's experience and care in construction are highly important for this problem.
Some material combinations are suitable in rather narrow velocity zones. The present summary basically only applies to water velocities between 0.5 and 12 ft/s (0.15 to 3.6 m/sec). What is said here cannot be compared directly with results from other fields such as pump, piping or condenser technology.
Various chemicals and other organic matters all have an influence – generally a detrimental one – either by direct action or because they assist or support the action of other elements. Sludge in the bottom of the pits helps produce H2S, de-aerates the water and supports bacteria. Petroleum and other greases can dissolve otherwise stable paints and seals.
All normally wet equipment should be welded with care.
Excess chlorine injection and concentrations due to insufficient mixing, are classical causes of corrosion on screens.
Sand and abrasive silts gradually destroy all protective coatings and the combination of abrasion and corrosion considerably speed up machine decay.
PROTECTION AGAINST CORROSION
Call BEAUDREY for help ; we have an unrivalled experience in this field.
The first thing to do is to assess how bad the problem actually is or might be. As each station is different and conditions change with the years, it is difficult to be sure of the best method of protection.
Painting is the first method to consider. It is generally the cheapest but soon becomes costly when the coating system becomes more sophisticated. Most paints are good, few are suited to screens and none are perfect.
Screens are subject to shocks and sometimes operate in the most unpleasant atmospheres with minimal care. Paint touch-up might be necessary after installation and during every outage.
The selected paint must be considered with several factors in mind: corrosion-resistance, shock-resistance, ease and efficiency of touching-up in damp conditions and aptitude to be adherent and crack-free on the many angles and tight corners encountered in screens. BEAUDREY can suggest systems for each case.
Paint performance essentially depends on the quality surface preparation prior to painting. Shot-blasting is essential.
Cathodic protection can be achieved either by using sacrificial anodes or impressed-current systems. Both types have advantages:
Impressed current is probably the most capable system. It must be well designed and carefully set in operation with enough time allowed for adjustments. It also needs regular, qualified inspection. Mistakes with impressed currents can lead to troubles far worse than what they should prevent, including stainless-steel corrosion, paint blistering, etc. It is definitely an item needing a maintenance contract by outside experts and a good technical environment.
BEAUDREY can, in cooperation with a world-renowned specialist, supply such a system to protect its machines.
Sacrificial anodes are easier to handle as too much does not hurt. Replacement is easy, yet sometimes forgotten in practice.
BEAUDREY can include sacrificial anodes in its supply. The normal design duration of the anodes is 2 years.
All parts of the machine that come out of the water cease to be protected. The most corroded areas are in the drawdown zone.
All parts of the machine which are in stagnant water or imprisoned in mud are subject to extreme corrosion particularly when they are made from stainless steel. The plant must be designed to avoid silting, mud concentrations and stagnant water areas.
Anode action is impaired by trash and growth.
These are by far the best when correctly chosen. For a screen, the choice of materials also depends on the shape, size and expected service life of the component. A slowly-corroding material is often enough for a wear part with a life expectancy of a few years.
In some cases, balance needs to be found between mechanical qualities and corrosion resistance. Perfection – when it exists – is often beyond economical reach. The required treatment to bring stainless steel to its highest corrosion-resistant status (curing, pickling, stress relieving, decontamination, etc.) must never by omitted.
Some plants cannot accept any corrosion at all (reverses osmosis, for instance) and alloys such as super duplex stainless steel must be used.
The name is deceptive as such alloys only resist corrosion in certain conditions. What follows only concerns the field of application of BEAUDREY machines.
Stainless steel corrosion protection is due to a very thin surface layer of oxide.
Corrosion resistance of stainless steel in our applications is a direct function of the alloy’s PREN number which is based on the chemical composition of the alloy. The higher the PREN number, the better the steel resists.
Martensitic alloys have next to no corrosion resistance (PREN < to 13). They can be used for mechanical purposes in fresh water.
Austenitic alloys are not suited for mechanical parts. AISI 304L (PREN 18) is suitable in fresh water. AISI 316L (PREN 24) is suitable in seawater but very sensitive to stagnant water. Sacrificial anodes are often added for protection.
Austenitic-ferritic, or duplex stainless steel (PREN 34) are much better and seldom require cathodic protection.
Super austenitic (PREN 41) is the best, affordable solution and is used when no metal oxide is acceptable downstream (reverses osmosis, for instance).
All duplex stainless steels are well suited for mechanical parts.
Stagnant water and H2S are the main causes of decay in stainless steel.
Corrosion resistance is only as good as the care taken during manufacture:
Overheating, weld burns, wrong anodes and mechanical overstressing must be avoided to prevent local destruction of the crystallographic structure on which corrosion resistance depends.
The fine oxide coating must be restored, and all iron or other pollutants must be removed by pickling and passivating the parts in chemical baths.
Aluminium bronze and cupro-nickel offer a very good resistance to corrosion but are very vulnerable to H2S. Owing to the high copper content, they have excellent anti-fouling properties. They are costly and require great manufacturing expertise.
BEAUDREY can help you select the materials best suited to your specific project.