Microbiologically Influenced Corrosion MIC - Whats is MIC?
Soft water is very corrosive; the reason for this can be thought of in several related ways. One way to think of this is that soft water has a lower pH, i.e. is more acidic, and therefore more able to cause corrosion. Another way to think of it is that soft water has ionic room and therefore wants to leech (dissolve) the metal from the pipe.
HydroFLOW allows a system to be run using hard rather than softened water, with all the consequent benefits for corrosion reduction. Of course, this benefit only occurs for systems that are changing from soft to hard water. Even if systems are already running on hard water (and dealing with the limescale by cleaning) a positive effect on corrosion is still present.
Below is a list of important terms when talking about corrosion:
The most common form of corrosion that takes place over large areas and is characterized by corrosion proceeding evenly over the entire surface area, or a large fraction of the total area. Dulling of a bright or polished surface, etching by acid cleaners, or oxidation (discoloration) of steel are examples of uniform corrosion.
Pitting corrosion is a cavity, hole or pit that forms in a small area or point. This kind of corrosion is very aggressive and is difficult to detect. The pits, or holes are obscured by a small amount of corrosion product (rust) on the surface. When a cathodic reaction in a large area (coating) sustains an anodic reaction in a small area (exposed metal), a pit, cavity or small hole will form. Oxidation occurs in the metal even when there is no supply of oxygen.
Crevice corrosion is a localized attack on a metal surface at, or immediately adjacent to, the gap or crevice between two joining surfaces in which a solution is trapped and not renewed.
Galvanic Corrosion is extraordinarily common and occurs when two metals with different electrochemical charges are linked via a conductive path. Corrosion occurs when metal ions move from the anodic metal to the cathodic metal. In this case, a corrosion-resistant coating would be applied to prevent either the transfer of ions or the condition that causes it. Galvanic corrosion can also occur when one impure metal is present. If a metal contains a combination of alloys that possess different charges, one of the metals can become corroded. This is known as intergranular corrosion. The anodic metal is the weaker, less resistant one, and loses ions to the stronger, positively charged cathodic metal. Without exposure to an electrical current, the metal corrodes uniformly; this is then known as general corrosion.
Stress Corrosion Cracking (SCC)
Stress Corrosion Cracking can seriously damage a component beyond the point of repair. When subjected to extreme tensile stress, a metal component can experience SCC along the grain boundary – cracks form, which are then targets for further corrosion. There are multiple causes of SCC, including stress caused by cold work, welding, and thermal treatment. These factors, combined with exposure to an environment that often increases and intensifies stress-cracking, can mean a part goes from suffering minor stress-corrosion to experiencing failure or irreparable damage. In brass, the failure due to stress corrosion cracking is dubbed “season cracking”; in steel, it is known as “caustic embrittlement.” Hydrogen embrittlement of steel is also considered to be a corrosion phenomenon.
General Corrosion occurs because of rust. When metal, specifically steel, is exposed to water, the surface is oxidized, and a thin layer of rust appears. Like galvanic corrosion, general corrosion is also electrochemical. To prevent oxidation, a preventative coating must interfere with the reaction.
Localized Corrosion occurs when a small part of a component experiences corrosion or encounters specific corrosion-causing stresses. Because the small “local” area corrodes at a much faster rate than the rest of the component, and the corrosion works alongside other processes such as stress and fatigue, the result is much worse than the result of stress or fatigue alone.
Caustic Agent Corrosion occurs when impure gas, liquids, or solids wear a material down. Although most impure gases do not damage the metal in dry form, when exposed to moisture they dissolve to form harmful corrosive droplets. Hydrogen sulfide is an example of one such caustic agent.
Microbiologically Influenced Corrosion (MIC)
Amongst the different possible corrosion mechanisms, microbiologically influenced corrosion is the one closely related to the activity of living microorganisms in the soil, including microalgae, bacteria, archaea, and fungi. Microbiologically influenced corrosion can take place in environments and working conditions where there is no other corrosion taking place, or it can take place in combination with other corrosion failures. More importantly, microorganisms can accelerate the kinetics of anodic/cathodic corrosion reactions, in such a way that they can be viewed as “catalytic” entities. Microorganisms are more known to induce a localized attack, including dealloying, pitting, localized galvanic corrosion, and stress corrosion cracking.
MIC, in any form, starts with the formation of a biofilm on the metal substrate. Bacteria cells get attached to the substrate of the material, where they grow, reproduce, consume nutrients and produce an extracellular polymer substance (EPS). Other metabolic products also generate a localized corrosion cell and collectively build the biofilm. It is also well known that microorganisms can use chemicals as nutrient sources and oxidize them.
Microbiologically Influenced Corrosion Damage Inside a Pipe
Microbial corrosion is divided into aerobic and anaerobic corrosions, with the latter known to induce a higher corrosion rate, while aerobic is associated with a lower corrosion rate. When it comes to bio-corrosion of metals, iron-reducing bacteria (IRB), iron and manganese-oxidizing bacteria, acid-producing bacteria (APB), and sulfate-reducing bacteria (SRB) are micro-organisms which are recognized to have more detrimental effects. IRB is known to be one of the main culprits that cause severe biodegradation of external metal surfaces. SRB induced biocorrosion on the interior of metal surfaces in conjunction with the low content of oxygen and stagnant liquid inside the pipe, provide desirable conditions for SRB to grow. In addition, SRB can appear under deposits of soil, water, hydrocarbons, chemicals, etc. The kinetics of corrosion in steels under the influence of micro-organisms, including SRB, can be up to ten times faster than the kinetics in the absence of micro-organisms. It is reported that SRB could get electrons directly from iron when there is no availability of organic carbon sources. This could occur when the soil acts as a barrier between the metal surface and the external environment.
To summarize - MIC deteriorates the metal surface through the metabolic activity of micro-organisms. The damage due to MIC is broken down into three steps: biofilm forms, changes of environment on the metal surface occurs, and finally the deterioration of the metal. The common bacteria associated with MIC are iron reducing bacteria (IRB), sulfate reducing bacteria (SRB), and acid-producing bacteria (APB). MIC damage results mainly in pitting corrosion, crevice corrosion, and stress corrosion cracking.
What is Magnetite?
Magnetite is the iron oxide Fe3O4 and refers to an ore of iron that occurs as a black isometric mineral of the spinel group. It is an important oxide of iron because it is highly polar and magnetic.
When HydroFLOW is applied to a pipe, the magnetite forms in a different manner. Instead of forming as a flakey substance, the magnetite forms as a hard, black layer on the pipe surface. This acts as a barrier between the iron in the pipe and the water (particularly the oxygen in the water) and slows further corrosion. In this sense it causes the magnetite to act like the oxides of other metals - for example, copper and zinc. Aluminum, when freshly cut, has a shiny appearance that soon fades to a dull grey. This is because a newly cut surface will expose aluminum, which then turns to aluminum oxide as it is exposed to the oxygen in the air.
Similarly, eventually, the orange of copper will (if left exposed to the outside elements) turn green. We usually do not think of these cases as ‘corrosion’, even though they are a metal turning into an oxide. This is because the process is self-limiting. The Statue of Liberty is a perfect example of this. She has not corroded away, even though she has turned green!
What is the Skin Affect?
To understand the skin effect and how it can reduce corrosion we have to discuss magnetic fields, what the magnetic field of a wire and a pipe looks like, how this affects the current, and how the current distribution can influence the corrosion rate.
HydroFLOW induces an AC current into the pipe, and we know that AC current mainly flows on the outside of a conductor (e.g. a pipe). Chemical reactions require electron transfer, and any free electrons are removed from the interior surface of the pipe by the skin effect through an electro-chemical reaction. Metal atoms in the pipe material become dissolved in the water. To do this they need to change from neutral atoms to charged ions - i.e. they need to lose an electron. The electron is then transferred to ions within the water. The skin effect, therefore, slows the chemical reactions that cause corrosion.
We first need to look at the magnetic field around a wire. Initially, let us simply consider a thin wire carrying a constant direct current (DC). The magnetic field is given by Ampere's Law, and it consists of a series of magnetic lines that wrap the wire. The direction of the field lines is given by the “right-hand grip rule".
The right-hand grip rule is as follows:
- make a fist, point your thumb in the direction of the current, and your fingers will show the direction of the magnetic field.
Inside the material of the pipe, we have a magnetic field. The important point is that the field is not static but changing. Now, a changing magnetic field induces an electric field. Which direction does this field operate in? Well, an induced electric field always acts to oppose the current that causes it. It is called a “back EMF" because it “acts back" to try to stop the current that caused it in the first place. In this respect, it acts as a resistance or a bit like a kind of friction.
The upshot is that away from the surface where the field is strong, the current feels a force opposing it. The force it feels is such that the current is exactly canceled. Near the surface, the field drops to zero and so there is no opposition to the current. Thus, we see that an AC current flowing through a conductor will tend to flow at the surface, with less current flowing in the center.
With a HydroFLOW unit in place, any electrons that are released into the pipe by the dissolution of the zinc are swept down the pipe by the HydroFLOW device. Then by the skin effect, they are forced to the outside of the pipe. This means that there are no electrons available for the chemical reactions to continue, blocking the reaction chain. This means the reaction cannot continue and therefore the corrosion slows.