Corrosion & It's Causes
The aim of corrosion protection measures is the exclusion of direct contact between water and the pipe or container walls. Even the much desired natural chalky-rust film, or the copper carbonate protective film, normal presuppose a previous guided corrosion process, depending upon the composition of the protective film, its thickness and structure.
These artificial protective films on metal surfaces in contact with water contain carbon. These films are impermeable to water as a result of the action of the phosphate mineral constituents and by raising the pH-value. Genuine lime-rust films without the involvement of phosphate components are rare in practice.
General Surface Corrosion
General surface corrosion involves uniform material removal from the metal surface. The speed at which the process occurs varies depending upon the material, its surface properties, carbonate hardness, the concentration of aggressive carbonic acid, chloride, nitrate and sulphate ions. This type of corrosion often results in rust deposits, but rarely in damage. Such corrosion can rarely be excluded without suitable treatment of the water. Materials exposed to general surface corrosion usually cause discolouration or turbidity of the water due to the corrosion products formed.
Pitting is the formation of a pin-like or scratched flaw in the metal surface that is in direct contact with water, even in the presence of an appropriate coating or protective film. The galvanized or copper pipe and container walls are ruptured in the final phase of this form of corrosion.
The causes of pitting are primarily:
a) Aeration cells resulting from deposits
Even though crystal clear water is supplied by the water works, pollutants can still occasionally be flushed in, incorporated when the pipes are installed, or formed as a result of bicarbonate decomposition and corrosion. These various types of coarse and fine-grain precipitates form "aeration cells" underneath the covered metal surface. There they initiate an electrical corrosion current that leads to pitting as a result of selective corrosion. Such cells can be created by rust particles, grains of sand, flaked off scale, residues of sealing material, soldering grease, brick chips, metal shavings, hemp pieces, cutting oil, scale corrosion products, etc.
b) Aeration cells resulting from gas bubbles
When water is heated gas bubbles are formed as the water is no longer able to hold the dissolved carbon dioxide, oxygen and nitrogen. Furthermore, there is a pressure drop in the system in the direction of the tap. An electrical corrosion current arises in the immediate vicinity of the gas bubbles that can also give rise to pitting.
c) Local cell formation
In instances where the water was in contact with copper-bearing materials, any copper ions that may have been dissolved are then precipitated as metallic copper on the zinc or iron surface, and this can also lead to pitting. Even a single element, be it as a result of adverse alloying constituents, unevenness, damage or deposits on the barrier firm, can result in the multitude of electrical corrosion currents over a wide heterogeneous mosaic-like area. Each of these can initiate pitting in the event of critical water or operating conditions.
d) Contact corrosion
Direct contact of two wet metals that deviate widely from one another in the electrochemical potential series can cause contact corrosion. The least refined metal of the two is dissolved with the result that several threads of one metal type can be literally eaten away.
The type and speed of corrosion are additionally influenced by different chemical and physical factors, the principal ones being:
a) Constituents of water
Carbon dioxide, carbonate hardness and pH value are of immense importance for all processes that take place in water. The chloride, nitrate and sulphate ions contained in the water generally promote corrosion. Salts with a basic reaction, such as silicates and phosphates, increase the pH-value. This, in conjunction with the carbonate hardness constituents, decides the quality of the protective film that is formed. Drinking water extracted from surface water can contain natural organic inhibitors whereas these are normally lacking in ground water. The lack of these natural organic inhibitors can result in increased pitting susceptibility of copper pipes.
b) Water temperature
Temperature is a decisive factor for the speed of corrosion. At temperatures exceeding 60C the corrosion-inhibiting effect of zinc is lost as a result of potential reversal. This can result in considerable pitting damage to bare iron surfaces and ungalvanized threads. The influence of radiation heat is often underestimated. This radiation may be due to inadequate insulation, adverse pipe laying conditions or temperature fluctuations in the hot-water pipe. Local overheating of the water at the heat exchange surfaces of water treatment equipment can significantly exceed the thermostat setting, thereby significantly accelerating bicarbonate decomposition.
c) Flowing rate
Low flowing rate and prolonged standstill times in the entire water supply system, and stagnation in pipes to taps that are seldom used, are all intensive corrosion promoting factors. This is also due to the fact that the mineral components and oxygen that are needed for the formation of a protective film are lacking.
d) Choice of material
The type and quality of material used, and the manner in which they are processed, all play a decisive role. Only immaculate materials and pipes in keeping with appropriate standards must be used.
e) Protective measures
Irrespective of the composition of the water and operating conditions. a backwash protective filter should be installed at the point of entry into the building, just after the water meter to retain foreign particles that could otherwise form aeration cells. If possible, the filter should be installed prior to the pressure test and before the water supply system is commissioned for service. Various mixtures of suitable inhibitors are used, depending upon the composition of the water. These include orthophosphate and polyphosphate, as well as components in mineral solution that correct the pH value. These mineral solutions, which are added by a dosing pump, form a protective film. The additional installation of a water softening system may be required if the water is hard.
This article is provided by Judo Water Treatment, a supplier of water filtration and softening products to residential, commercial and industrial markets.