 |
| Add caption |
Within the jacket water system a number of corrosion cells are available but the two most common and most damaging are due to dissimilar metals and differential aeration. In both types of cell there exists an anode and a cathode, the metals which form part of the jacket system, and an electrolyte which is the cooling water. The rate at which corrosion takes place is dependent upon the relative areas of the cathode and the anode and the strength of the electrolyte. It is the anode that wastes away. Corrosion due to temperature differences is avoidable only by the use of suitable treatments. Dissimilar metals-a galvanic cell is set up where two different metals and a suitable liquid are connected together in some way. All metals may be placed in an electro-chemical series with the more noble at the top . Those metals at the top are cathodic to those lower down. The relative positions between two metals in the table determined the direction and strength of electrical current that flows between them and hence, the rate at which the less noble will corrode
Galvanic Action
Corrosion within cooling systems can occur if the coolant, i.e. water, has not been properly treated. The corrosion can take the form of acid attack with resultant loss of metal from a large area of the exposed surface, or by Oxygen attack characterised by pitting. A primary motive force for this corrosion is Galvanic action
The Galvanic Series.
Or Electromotive series for metals
Cathode
Gold and Platinum
Titanium
Silver
Silver solder
Chromium-Nickel-Iron (Passive)
Chromium-Iron (Passive)
Stainless Steel (Passive)
Copper
Monel
70/30 Cupro-Nickel
67-33 Nickel-Copper
Hydrogen
lead
Tin
2-1 Tin lead Solder
Bronzes
Brasses
Nickel
Stainless-Steel 18-8 (Active)
Stainless Steel 18-8-3 (Active)
Chromium Iron (Active)
Chromium-Nickel-Iron (Active)
Cadmium
Iron
Steel
Cast Iron
Chromium
Zinc
Aluminium
Aluminium Alloys
Magnesium
Anode
The metals closer to the anodic end of the list corrode with preference to the metals towards the cathode end.
A galvanic cell can occur within an apparently Homogeneous material due to several processes on of which is differential aeration where one area is exposed to more oxygen than another. The area with less oxygen becomes anodic and will corrode.
Galvanic action within metal
Close up ofmetal plate
Galvanic action due to temperature gradient
Effect of temperature on metal
This situation can exist in cooling water systems with complex layout of heat exchangers and passage ways within the diesel engine. Systems containing readily corrodible metals such as zinc, tin and lead alloys can complicate and intensify problems by causing deposit formations.
Differential Aeration
-Where only a single metal exists within a system corrosion can still take place if the oxygen content of the electrolyte is not homogenous. Such a situation can occur readily in a jacket water system as regions of stagnant flow soon have the oxygen level reduced by the oxidation of local metal. The metal adjacent to water with reduced levels of oxygen become anodic to metals with higher oxygen content electrolyte in contact with it.. Generally, the anodic metal is small in comparison the cathode i.e. the area of stagnant flow is small compared to the area of normal flow of electrolyte, and high rates of corrosion can exist. One clear case of this is the generation of deep pits below rust scabs.
Solutions
Water treatment
To remove the risk of corrosion it is necessary to isolate the metal surface form the electrolyte. One method would be by painting, but this is impractical for engine cooling water passages. A better solution would be a system which not only searched out bare metal coating it with a protective barrier, but also repaired any damage to the barrier.
for corrosion to occur four conditions must be met;
There must be an Anode
There must be a cathode
An electrolyte must be present
An electron pathway should exits
Corrosion Inhibitors
Corosion inhibitors are classified on how they affect the corrosion cell and are placed into three catagories;
Anodic Inhibitors
Cathodic Inhibitors
Combination inhibitors/organic inhibitors
Common Corrosion Inhibitors
Principally Anodic Inhibitors
Chromate
Nitrite
Orthophosphtae
Bicarbonate
Silicate
Molybdate
Principally Cathodic Inhibitors
Carbonate
Polyphosphate
Phosphonates
Zinc
Both Anodic and Cathodic Inhibitors
Soluble Oils
Mercaptobenzothiazole (MBT)
Benzotriazole (BZT)
Tolytriazole (TTZ)
Anodic Inhibitors
Nitrite (NO2- )- These are the most commonly used form of treatment and operate by oxidising mild steel surfaces with a thin, tenacious layer of corrosion product (magnetite Fe3O4). Relatively high volumes of treatment chemical are required so this method is only viable on closed systems
Sodium Nitrite- (sometimes with Borate added)-effective with low dosage, concentration non-critical. It is non toxic, compatible with anti freezes and closed system cooling materials. It does not polymerise or breakdown. However protection for non-ferrous materials is low. An organic inhibitor is thus required. Although will not cause skin disease it will harm eyes and skin. Approved for use with domestic fresh water systems.
Sodium Nitrite is a Passivator, a passivator will act chemically to produce an insulating layer on the metal surface. Whenever corrosion takes place the corrosion products including bubbles of gas, are released from the metal surfaces. Passivating chemicals act on the corrosion products preventing release from the metal surface and thus stifling further corrosion. If the insulating layer becomes damaged, corrosion begins a gain and the passivator acts on the new products to repair the layer.
Chromate's-the first passivator product was Sodium Chromate which was an excellent inhibitor. Inexpensive, effective and concentration easily tested. Corrosion may increase by incorrectly dosing, dangerous to handle, poisonous and can cause skin disease. Not allowed where domestic water production is in use (Jacket water heated evaporators). Unfortunately it was also highly toxic, a severe pollutant and staining agent, was imcompatible with antifreeezes, nd will attack zinc and soft solder slightly. Due to its toxicity is sometimes used as a biocide in such places as brine in large Reefer plants.
Silicates- react with dissolved metal ions at the anode. The resultant ion/silicate complex forms a gel that deposits on anodic sites. This gel forms a thin, adherent layer that is relaitvely unaffected by pH in comparsion to other inhibitors. The inhibiting properties increase with temperature and pH, normal pH levels are 9.5 to 10.5.
Care should be taken with the use of silicates, which are often used for the protection of systesm containing alumiinium. In the event of boiling increased concnetrations and lead to aggressive corrosion due to the high pH.
Orthophosphate Forms an insoluble complex with dissolved ferric ions that deposit at the anodic site. It is more adherent and less pH sensitive than other anodic inhibitors. The film forms in pH of 6.5 to 7.0. Dosage is typically 10ppm in neutral water
Cathodic Inhibitors
Polyphosphate- Forms complexes with Calcium, Zinc and other divalent ions, this creates positively charged colloidal particles. These will migrate to the cathodic site and precipitate to form a corrosion inhibiting film. The prescence of calcium is required at a typical minimum concentration of 50ppm.
Extreme variations in pH can upset the film and a reversion to orthophosphate will occur with time and temperature.
Positively charged zinc ions migrate to the cathodic site and react with the free hydroxyl ions to form a zinc hydroxide stable film at pH 7.4 to 8.2. If the water is too acidic the film will dissolve and not reform. If it is too alkaline the zinc hydroxide will precipitate in bulk and not at the cathodic site.
PhosphonatesInitially introduced as scale inhibitors to replace polyphosphates, they exhibit absorbtion at the metal surface especially in alkaline hard water. Generally used with other inhibitor types
Both Anodic and Cathodic Inhibitors
Benzotriazole and Triazole Specific corrosion inhibitor for copper. They break the electrochemical cirsuit by absorbing into the copper surface.
They are generally added to standard treatments.
Sollble and dispersible oils.Petroleum industry recognised that emulsifying cutting oils (erroneously called soluble oils) were able to reduce corrosion on metals by coating the surface. There were disadvantages though, if the coating became too thick then it could retard the heat transfer rate. Adherent deposits form as organic constituents polymerise or form break down products which can accumulate and disrupt flow. MAN-B&W recommend it not to be used.
It is effective in low dosages, safe to handle and safe with domestic water production. Effectiveness is reduced by contamination with carbon, rust, scale etc. Difficult to check concentration, overdosing can lead to overheating of parts
Oils are classed as a barrier layer type inhibitor. The surfaces being coated in a thin layer of oil.
Modern treatment
Nitrite-Borate treatment is most effective with a high quality water base. This treatment has no scale prevention properties and its effectiveness is reduced by high quantities of dissolved solids.
A modern treatment will be a Nitrite -Borate base, with a complex blend of organic and inorganic scale and corrosion inhibitors plus surfactants, alkali adjusters, dispersants and foam suppressers. A high quality water supply is still strongly recommended.
The Use of Sacrificial Anodes
-Electrolytic protection for the whole system by the use of sacrificial anodes is impractical. Parameters such as water temperature, relative surface area of anode and cathode, activity of metals in system and relative positions in galvanic series come into play. Anodic protectioon has become out of favour for cooling water systems as it can lead to local attack, causes deposits leading to flow disturbance and it has no scale protection
Preparation for cooling water treatment
-All anodes should be removed and the system inspected. No galvanised piping is to be used (old piping can be assumed to have had the Galvanising removed). High quality water should be used and chemicals measured and added as required. A history log should be kept
Microbiological Fouling
Under certain conditions bacteria found in cooling water systesm can adapt to feed on the nitrite treatment.This can lead to rapid growth, formation of bio-films, fouling and blocakages.
Typical evidence is a loss of nitrite reserve but a stable or rising conductivity level as the nitrate formed still contributes to the conductivity,
Problems of this sort are rare due to the ellevated temerpatures and pH levels. SHould it occur treatment with a suitable biocide is required.
No comments:
Post a Comment