Engineering Chemistry

Lesson 8. Prevention methods of corrosion

8.1  METAL PURIFICATION

The corrosion resistance of a pure metal is usually better than that of one containing impurities or small amounts of other elements. However, pure metals are usually expensive and are relatively soft and weak. In general, this category is used in relatively few cases which are more or less special. Aluminium is a good example because it is not expensive in a fairly pure state 99.5% plus. The commercially pure metal is used for handling hydrogen peroxide, where the presence of other elements may cause decomposition because of catalytic effects. In another case, localized attack of aluminium equipment occurred because of segregation of impurity iron in the alloy. Reduction of the maximum iron content, agreeable to both producer and user eliminated the localized attack and satisfactory performance of the equipment was obtained without added cost of material. Another example is arc-melted zirconium, which is more corrosion resistant than induction-melted zirconium because of more impurities in the latter. This is a special case in an atomic-energy application where a little corrosion is too much.

 

8.2  NON METAL

This category involves solid non-metallic construction and also sheet linings or coverings of substantial thickness (to differentiate from paint coatings). The five general classes of non-metallic are:

1. natural and synthetic rubbers
2. plastics
3. ceramics
4. carbon and graphite
5. wood

In general, rubbers and plastics, as compared with metals and alloys, are much weaker, softer, more resistant to chloride ions and hydrochloric acid, less resistant to strong sulphuric acid and oxidizing acids such as nitric, less resistant to solvents, and have relatively low temperature limitations (75 to 90^oC). Ceramics possess excellent corrosion and high-temperature resistance, with the main disadvantages being brittleness and lower tensile strength. Carbons show good corrosion resistance, electric and heat conductivity, but they are fragile. Wood is attacked by aggressive environments. Rubber linings for tanks, lines, fans, filters, scrubbers, etc. This manual contains sections on rubbers used, selection, testing, and design, and fabrication, preparation of equipment to be lined, processing, inspection, acceptance, maintenance, chemical resistance and cost factors. Good and bad practices are described. Hard and soft natural rubbers, polychloroprene and butyl rubbers are included.

 

8.3 ALTERATION OF THE ENVIRONMENT

8.3.1 Changing Mediums altering the environment provides a versatile means for reducing corrosion. Typical changes in the medium that are often employed are (1) lowering temperature, (2) decreasing velocity, (3) removing oxygen or oxidizers, and (4) changing concentration. In many cases, these changes can significantly reduce corrosion, but they must be done with care. The effects produced by these changes vary depending on the particular system.

 

8.3.2 Lowering temperature: This usually causes a pronounced decrease in corrosion rate. However, under some conditions, temperature changes have little effect on corrosion rate. In other cases, increasing temperature decreases attack. This phenomenon occurs as hot, fresh or salt water is raised to the boiling point and is the result of the decrease in oxygen solubility with temperature. Boiling sea water is therefore less corrosive than hot sea water (e.g., 65^oF).

 

8.3.3 Decreasing velocity: This is often used as a practical method of corrosion control. Velocity generally increases corrosive attack, although there are some important exceptions. Metals and alloys that passivate, such as stainless steels, generally have better resistance to flowing mediums than stagnant solutions. Very high velocities should be always avoided where possible, because of erosion-corrosion effects.

 

8.3.4 Removing oxygen or oxidizers: This is a very old corrosion control technique. Boiler feed water was de-aerated by passing it through a large mass of scrap steel. In modern practice this is accomplished by vacuum treatment, inert gas sparging, or through the use of oxygen scavengers. Hydrochloric acid that has contacted steel during its manufacture or storage contains ferric chloride as an oxidizer impurity. This impure acid, termed muriatic acid in commerce rapidly corrodes nickel-molybdenum alloys, whereas these materials possess excellent resistance in pure hydrochloric acid. Although de-aeration finds widespread application, it is not recommended for active-passive metals or alloys. These materials require oxidizers to form and maintain their protective films and usually possess poor resistance to reducing or non-oxidizing environments.

 

8.3.5 Changing concentration: Decreasing corrosive concentration is usually elective. In many processes, the presence of a corrosive is accidental. For example, corrosion by the water coolant in nuclear reactors is reduced by eliminating chloride ions. Many acids such as sulphuric and phosphoric are virtually inert at high concentrations at moderate temperatures. In these cases, corrosion can be reduced by increasing acid concentration.

No discussion of corrosion control would be complete without mentioning the magic devices or water conditioning gadgets that have been and continue to be widely sold for purposes of controlling water corrosion. These gadgets are usually promoted on the basis that they will stop corrosion, prevent scaling, destroy bacteria, and improve taste and odour, or reduce water hardness. Some manufacturers make all of the above claims for their product. In every case, the device is based on some pseudoscientific principle, is simply constructed, quite expensive, and totally worthless. Several of them consist merely of a pipe coupling that looks identical to those available in any hardware store.

Magic devices should not be confused with the water-softening, water-treating and cathodic protection apparatus and systems sold by reputable manufacturers. The worthless device is easily spotted by a number of clues:

(1 ) it is based on a questionable or a "secret" new principle. (2) The advertising contains an excessive number of testimonials. (3) The promotion makes no mention of any limitations the device will work in any kind of water and protect any size system. (4) The device is always sold with a complete guarantee.

 

8.4  INHIBITORS:

An inhibitor is a substance that, when added in small concentrations to an environment, decreases the corrosion rate. An inhibitor can be considered as a retarding catalyst. There are numerous inhibitor types and compositions. Most inhibitors have been developed by empirical experimentation, and many inhibitors are proprietary in nature and thus their composition is not disclosed. Inhibition is not completely understood because of these reasons, but it is possible to classify inhibitors according to their mechanism and composition.

Adsorption-type inhibitors: These represent the largest class of inhibiting substances. In general, these are organic compounds which adsorb on the metal surface and suppress metal dissolution and reduction reactions. In most cases, it appears that adsorption inhibitors affect both the anodic and cathodic processes, although in many cases the effect is unequal. Typical of this class of inhibitors are the organic amines.

 

8.5 CATHODIC AND ANODIC PROTECTION

8.5.1 Cathodic Protection

Cathodic protection was employed before the science of electrochemistry had been developed. The principles of cathodic protection may explained by considering the corrosion of a typical metal M in an acid environment. Electrochemical reactions occurring are the dissolution of the metal and the evolution of hydrogen gas;

                                          M → M⁺ⁿ  +  ne

                                         2H⁺   +  2e  →H₂

Cathodic protection is achieved by supplying electrons to the metal structure to be protected. Examination of above equations indicates that addition of electrons to the structure will tend to suppress metal dissolution and increase the rate of hydrogen evolution. If current is considered to flow from (+ ) to ( - ), as in conventional electrical theory, then a structure is protected if current enters it from the electrolyte. Conversely, accelerated corrosion occurs if current passes from the metal to the electrolyte. This current convention has been adopted in cathodic protection technology and is used here for consistency.

There are two ways to cathodically protect a structure : ( 1) by an external power supply or, (2) by appropriate galvanic coupling.  An external dc power supply is connected to an underground tank, and the negative terminal of the power supply is connected to the tank, and the positive to an inert anode such as graphite. The electric leads to the tank and the inert electrode are carefully insulated to prevent current leakage. The anode is usually surrounded by backfill consisting of coke breeze, gypsum or bentonite, which improves electric contact between the anode and the surrounding soil. Current passes to the metallic structure, and corrosion is suppressed.

Cathodic protection by galvanic coupling: Magnesium is anodic with respect to steel and corrodes preferentially when galvanically coupled. The anode in this case is called a sacrificial anode since it is consumed during the protection of the steel structure. Cathodic protection using sacrificial anodes can also be used to protect buried pipelines. The anodes are spaced along the pipe to ensure uniform current distribution. Protective currents are usually determined empirically.  Aggressive corrosives such as hot acids require prohibitively high currents, whereas much lower currents are needed to protect steel in less severe environments. For example, in certain very acidic soils, 10 to 15 mA is often needed to reduce the corrosion of steel structures to tolerable levels. Also, pipes with organic coatings require much lower currents since the only areas requiring protection are defects in the protective layer. In such cases, trial-and-error adjustments of anode size or applied current can be made until satisfactory protection is achieved. A more accurate and less time-consuming approach is to measure the potential of the protected structure with a suitable reference electrode.

 

8.5.2 Anodic Protection

In contrast to cathodic protection, anodic protection is relatively new. This technique was developed using electrode kinetics principles. Anodic protection is based on the formation of a protective film on metals by externally applied anodic currents. It appears that the application of anodic current to a structure should tend to increase the dissolution rate of a metal and decrease the rate of hydrogen evolution. This usually does occur except for metals with active-passive transitions such as nickel, iron, chromium, titanium, and their alloys. If carefully controlled anodic currents are applied to these materials, they are passivated and the rate of metal dissolution is decreased. To anodically protect a structure, a device called a potentiostat is required. A potentiostat is an electronic device that maintains a metal at a constant potential with respect to a reference electrode. The potentiostat has three terminals, one connected to the tank, another to an auxiliary cathode (a platinum or platinum-clad electrode), and the third to a reference electrode (calomel cell). In operation, the potentiostat maintains a constant potential between the tank and the reference electrode. The optimum potential for protection is determined by electrochemical measurements. Anodic protection can decrease corrosion rate substantially.

 

8.6 ORGANIC COATINGS

These involve a relatively thin barrier between substrate material and the environment. Paints, varnishes, lacquers, and similar coatings protect more metal on a tonnage basis than any other method for combating corrosion. Exterior surfaces are most familiar, but inner coatings or linings are also widely utilized. These coatings should not be used where the environment would rapidly attack the substrate material.

Aside from proper application, the three main factors to consider for organic coatings, are (1) surface preparation, (2) priming coat, and (3) selection of top coat. If the metal surface is not properly prepared, the paint may peel off because of poor bonding. If the primer does not have good adherence or is not compatible with the top coat, early failure occurs. If the first two factors are wrong, the system will fail regardless of the top coat used. Poor paint performance is, in most cases, due to poor application and surface preparation.

Surface preparation involves surface roughening to obtain mechanical bonding (teeth) as well as removal of dirt, rust, mill scale, oil, grease, welding flux, crayon marks, wax, and other impurities. In other words a clean, rough surface is needed. The best method is to grit-blast or sandblast the steel surface. Other methods are pickling and other types of chemical treatments, scraping, wire brushing, flame cleaning (heat with torch and scrape off dirt and scale), chiselling, and chipping. Pinholes in welds and sharp edges should be ground out to ensure contact between the paint and the metal. Other chemical methods are solvent degreasing, hot or cold alkali treatments, phosphatising, chromate treatment and electro chemical treatments such as anodizing and cathodic cleaning.