LONGMONT personnel have been involved in a variety of Cathodic engineering projects ranging from Deep-well ground bed installation through pipeline and wharf installation, testing and commissioning.
In conjunction with our clients and associated design house we design, install, test, and commission a variety of Cathodic Protection systems. Integration of systems with Lightning Protection, Electrical Design and Telecommunications system, and statutory earthing in hazardous areas is speciality service we offer.
With experience in Electrical Supply Authority inspections, Electrical Safety is our primary concern and business focus. Through this, we ensure integrated compatibility of systems.
A Brief Outline of Corrosion
Discussion Paper by Terry Mulligan MIEEE MACA 2002
The following discussion is in general terms only. The complexity of the association of elements related to corrosion makes evaluation of any give problem a complex matter and at times an apparently inexact science. This discussion identifies only some of the salient points associated with evaluation and prevention of "Corrosion".
Corrosion
Metals such as Aluminium and Steel are manufactured through a refining process that imparts a tremendous amount of energy to the metals to remove the impurities. [So high are the levels of energy are required for the smelting of Aluminium that it is often referred to as ‘congealed electricity'.] Once the impurities are removed, other components are added to give the final product the characteristics necessary for a specific application. It is largely this energy, imparted during manufacture, which lends the metal its propensity to corrode or return to the natural state or lower energy level. This levelling is referred to as entropic equalisation.
There are a number of mechanisms that allow the metals to corrode, including corrosive elements such as salts in water, varying concentrations of oxygen in water, temperature variations, the presence of acids and sulphates or even the presence of acid producing bacteria. Regardless of the assisting mechanism, corrosion is electrochemical in nature, involving anodic and cathodic regions on a metal surface. The region is formed by connecting dissimilar metals and in the presence of an electrolyte [water with dissolved salts] also connecting the two metals; a corrosion cell comes into being.
All corrosion cells consist of four distinct components;
- An anode,
- A cathode,
- Continuous electrolyte between the two, &
- An electrically conductive path, [usually the metal surface of the anode and cathode connected together.]
The anodic region of the metal erodes [corrodes / rusts]. See Sketch 1.
Electric current generated by the corrosion cell is discharged from the exposed metal surface at the anodic region, [where the surface corrosion occurs] and flows through the electrically conductive electrolyte to where it is received by the exposed metal surface in the cathodic region. The electrically conductive metal between the 'Anode' and the 'Cathode' completes the circuit, with hydrogen gas vented to atmosphere, chlorides being deposited out in the oxidate residue.
Illustration - Corrosion of Metal - Indicative of current movement between Anodic and Cathodic Areas through the Electrolyte. The more conductive the Electrolyte [e.g. more dissolved salts, chlorides or acids], the higher rate of current movement & more accelerated the rate of corrosion.
The rate of corrosion is a function of the 'Corroding Current'. This current is dependent on the voltage that is causing the current to flow; therefore, increasing the driving voltage accelerates the metal propensity to corrode or increases the rate of corrosion. Connecting one metal to another metal that has a greater manufacturing energy content increases the driving voltage of the corrosion cell.
Any 'Galvanic Cell' corrosion has five factors that determine the rate at which the elements will equalize their charge. These are;
- The potential difference between the two metals,
- The ration of exposed areas of the two metals,
- The resistance of the electrolyte,
- The resistance of the connection circuit, [Connection cable, metal to metal, damp concrete etc.]
- Stray current between the metals, pipes, conductors, or associated structures.
The relationship of the various metals and the voltage difference is listed on the anodic scale, some elements of which are listed in table 1 below.
Table 1: Galvanic Series (See also the note below)
Note: In a given environment (one standard medium is aerated, room-temperature seawater), one metal will be either more noble or more active than the next, based on how strongly its ions are bound to the surface. Two metals in electrical contact share the same electron gas, so that the tug-of-war at each surface is translated into a competition for free electrons between the two materials. The noble metal will tend to take electrons from the active one, while the electrolyte hosts a flow of ions in the same direction. The resulting mass flow or electrical current can be measured to establish a hierarchy of materials in the medium of interest. This hierarchy is called a Galvanic series and can be a very useful design guideline when choosing materials for construction.
To identify the driving voltage of a 'Galvanic Circuit', the relative positions in the electromotive series can provide the simple answer; i.e.
- The difference between copper and iron is 0.38 Volts DC;
- The difference between steel and aluminium is 1.7 Volts DC;
- The difference between copper and aluminium is 2.04 Volts DC.
By treating the resistance of the electrolyte as negligible, the current in the circuit can be calculated from the relative exposed surface areas of the dissimilar metals and provide a cell 'Rot Rate' or rate of corrosion. Typically a DC current of one amp flowing for one year will corrode [rot] away 9 Kg of steel, 10Kg of copper, 11.8 Kg of zinc, 22 Kg of lead or 3.5 Kg of aluminium. The eroded material is returned to the universally present electrolyte.
Secondary features of corrosion cell action is that elevated temperature will result in an increased rate of corrosion, as will the impurities in any water / electrolyte modify the cathodic / anodic relationship. For example in the presence of water borne sulphates [hard water] at elevated temperatures [500C+] copper becomes anodic with respect to steel, in many cases causing copper pipes in heat exchangers or water heating systems to corrode & leak due to internal corrosion("Metals Handbook" refers, see Bibliography below).
Bibliography;
1. AS 2832: 1998 Cathodic Protection of Metals
2. IEEE Standard 142 - 1991 - Connection to Earth
3. Cathodic Protection - L.G Rankin, "The Indonesian Pipeliner October 1999"
4. Metals Handbook - Volume 13, Corrosion. 9th Edition ASM International, Metals Park OHIO, (1987)
