Metal Corrosion: Causes and Means of Protection

Corrosion is determined by objective laws of nature which cannot be stopped. Insufficient attention to the problem of corrosion can cause not only considerable financial loss, but also great misadventures. Therefore, water pipes, long-distance heating, gas pipelines or power lines need particular attention.

Metal corrosion is due to Objective Reasons

Corrosion is a spontaneous metal decomposition that occurs through chemical and electrochemical interaction of a metal and environment. Methods of protection against corrosion included in the international standards allow reducing losses caused by corrosion by 10-15%, and another 10% may be reduced using technical experience of specialists in the field of corrosion.

In the construction sector, serious problems are posed by electrochemical corrosion, which is an electrochemical process resulting in macro-galvanic and micro-galvanic elements. On a metal surface, small anodic and cathodic areas are formed, electric current appears, and corrosion products may be formed in areas quite distant from its focus. Several types of corrosion are distinguished in scientific literature: atmospheric corrosion, underground metal structure corrosion, microbial corrosion, and corrosion caused by mechanical impact.

The most commonly recommended methods of protection against atmospheric corrosion are properly selected metal coatings and alloying. To protect underground metal structures against corrosion, it is particularly important to take appropriate measures, and to take into account the corrosivity of the soil and groundwater activity. Usually metal structures are covered with insulating materials.

There are not any special methods to alleviate microbial corrosion, i.e., caused by iron bacteria. Experts recommend simply avoiding anaerobic conditions. It is recommended to pour gravel and lime with a higher pH level around pipes, or to use properly selected construction coatings.

The biggest concern among builders and engineering systems installers is caused by corrosion incited by the so-called stray electric currents. Their distribution in the ground or a structure, intensity and exposure time depend on many factors, which change sharply over time. Underground metal structures or pipelines in a building are affected by electric currents which cause an intense corrosion process. Therefore, builders consider using only grounded AC and DC electrical installations.

If a cathodic protection device is employed, currents from the anode grounding spread through the soil and flow into the protection device. In order to protect long pipes, anode grounding is often installed several kilometres away. Such a distant location of anodic grounding results in maximum stray current density occurring in anode grounding. Operating cathode devices may also cause bigger stray currents.

In case of cathodic protection of poorly insulated pipes by several devices, the current in the ground may reach tens or even hundreds of amps. Flowing in the ground and being exposed to the underground metal structures, stray electrical currents spread therein as the structures feature a much smaller resistance. Metal corrodes in anode area.

Steel and cast iron pipes are often isolated hermetically from each other using rubber gaskets. However, if the potential reduction in the pipe is high, stray currents may jump over the insulating material and continue to flow, and an intense corrosion will take place in the anode area of the pipe. The main goal of protection against stray currents is to prevent their release from metal and thus eliminate the anodic decomposition of metal.

Focus on Corrosion in the Design Stage

Many corrosion-related issues could be resolved in the design stage, because the construction durability depends on the appropriateness of the chosen design solution. If it is not appropriate or the system is installed inadequately, this may create favourable conditions for the corrosion process.

During the design stage it is necessary to consider proper distances between pipelines and electricity networks, and the power leaks from disorderly power sources through the concrete or other structures. It is also important to avoid the creation of powerful magnetic fields near the pipeline, as they can stimulate inductive currents. A proper system operation is equally important. In some cases, electric equipment is connected inappropriately, and pipelines are used for grounding. Some people use water pipes for grounding of washing machines.

Conditions for Corrosion in Water Plants

As already mentioned, the main forms of corrosion in water pipe systems are internal and external corrosion. The most common internal corrosion may affect all kinds of metal pipes. Several conditions are required for corrosion to occur: a metal piping system which circulates water and the presence of oxygen in the system. The interaction of metal, oxygen and water causes corrosion, which consists of anode and cathode processes on metal surfaces and connections between two or more types of metal. Anode reaction is simply a metal decomposition, when the metal is disintegrated into metal ions. The cathode process is a reaction between oxygen, water and electrons released during the anode process. Anode and cathode reactions occur for a variety of reasons: scratches, cracks, small differences in metal alloys, etc.

Corrosion in Different Types of Pipes

All types of metal pipes may corrode. Corrosion of galvanized steel pipes is caused by water quality, the presence of copper in water, mud (anaerobic bacteria) and higher than the 70-degree water temperature. Hot-dip galvanized steel pipes may be used if the water hardness does not exceed 4 °dH or if it does not contain acids forming aggressive lime-scale residues. If the water does not meet these conditions, this will first of all result in zinc decomposition, and then the disintegration of steel, which will be accompanied by visibly brown water. Of course, it will take several years for a complete corrosion – and cavities – to appear. However, if the pipe is defective and it has scratches, the corrosion process may be very rapid, and it will take less than a year to occur. It is noted that this type of corrosion is 3-4 times more common in hot water pipes.

The influence of copper on inducement of the corrosion process is also important. Although water provided by water supply companies does not contain copper, a small amount of copper may enter water from copper tanks, pipes or reservoirs. Nevertheless, the water which previously flowed through copper pipes or was contained in copper reservoirs should not enter hot-dip galvanized steel pipes.

Particulate debris and deposits can also cause corrosion, especially on horizontally mounted pipes and at the bottom of connections. Copper may enter cooling mixtures because of melting of copper tubes in fan coil units. This will result in deposits and stimulate the formation of localized corrosion foci in areas of copper deposition. Such particles are typically formed by rust or dirt, impurities from other parts of the system, calcium, magnesium, silicon derivatives, e.g., when a certain part of an old pipe is replaced. This can be solved by installing special filters.

Steel pipes are coated with zinc to isolate metal mechanically, i.e., to protect it against aggressive environment, and electrochemically in case of surface damage. In this case corrosion occurs in a surface metal, which is more active. Zinc coatings are durable in hard water, wherein insoluble zinc carbonate is formed. As a result, the zinc surface becomes passive. It should be noted that zinc protects steel from corrosion only if the temperature does not exceed 50 degrees. Thus the issue of selecting suitable pipes, as the most favourable condition for Legionella to multiply is the water temperature ranging from 20 to 45 degrees. Hot water temperature should be 50-60 degrees, and the cold water should be below 25 degrees.

Copper pipes are considered to be highly resistant to corrosion, but it can occur for other reasons, e.g., turbulence, wear, scratches, or flux. The turbulence-related corrosion usually occurs due to excessive flow rate in pipes. Therefore, this problem is more common in hot water recirculation systems. It is recommended not to exceed a flow rate of 0.5 litres per second. In particularly poor conditions, turbulence corrosion can occur due to brown water, next to inappropriate connections, beside valves and solder joints or due to the accumulated air.

Due to pipe expansion strain, the wear-related corrosion often occurs in line with internal corrosion. To avoid this, when designing and installing the pipe it is necessary to direct its expansion in an appropriate direction. Pipe expansion can be compensated by correct positioning of the controls and fixed brackets.

Flux excess in a pipe can cause pitting corrosion. When soldering with flux, it is recommended to apply it at the end of the pipe only, and not on the inside of the connection. Flux residues must be cleaned before starting the system operation.

Stainless steel pipes are highly resistant to internal corrosion, but it is not recommended to connect them by soldering or welding as this may cause internal corrosion. For such pipes, it is advisable to use mechanical connections. The essential factors of sewage pipeline corrosion are oxygen, humidity, temperature, acids, bases, degrading aggressive liquids and a connection method.

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