Corrosion Prevention

By retarding either the anodic or cathodic reactions the rate of corrosion can be reduced. This can be achieved in several ways :
4.3.1 Conditioning the Metal
This can be sub-divided into two main groups:
(a) Coating the metal, in order to interpose a corrosion resistant coating between metal and environment. The coating may consist of:
(i) another metal , e.g. zinc or tin coatings on steel,
(ii) a protective coating derived from the metal itself,
e.g. aluminium oxide on “anodised” aluminium,
(iii) organic coatings, such as resins, plastics, paints, enamel, oils and greases.
The action of protective coatings is often more complex than simply providing a barrier between metal and environment. Paints may contain a corrosion inhibitor
ALSO……
Underbody structural components are typically coated to provide a first line of defense against corrosion. For light truck frames, the two most common coatings are hot melt wax and electrocoat (E-coat). Paints are also used on current light truck frames. Conversion coatings enhance the adhesion of electrocoat or paint, and they are commonly used in conjunction with these two coating types. Many underbody structural components, such as front rails on passenger cars, are made from sheet steel pre-coated with a metallic coating, e.g., galvanized or galvanneal sheet steel. Autophoretic and powder coatings are also used on underbody structural components.

*Coating type : 1. Internal Lining

Fig (4-1) internal coat

Description:
Internal coating using a two component liquid epoxy based paint.
Features:
This coating system has excellent anti-friction properties and good resistance to chemicals.

2. Fusion Bonded Epoxy (FBE) Powder Coating

Fig (4-2) Fusion Bonded Epoxy (FBE) Powder Coating

Description:
Stand alone coating system.
Features:
This coating system has adequate mechanical properties and effective anti-corrosion properties with resistance to high temperature operating service up to 120°C depending on raw materials used.

3. Dual Fusion Bonded Epoxy (D-FBE ) coating

Fig (4-3) Dual Fusion Bonded Epoxy (D-FBE ) coating

Description:
2-layer coating system composed of FBE primer (first layer), FBE topcoat (top layer).
Features:
This coating system has good mechanical properties and effective anti-corrosion properties and resistance to high temperature operating service up to 110°C or 150°C depending on raw materials used.

4. Bitumen / Asphalt Enameln (AE) Coating

Fig (4-4) Bitumen / Asphalt Enameln (AE) Coating

Description:
Multi-layer coating system composed of synthetic primer (first layer), enamel / inner wrap / enamel layer(s) (second and, if any following layers) and outer wrap layer (top layer).
Features:
This coating system has adequate mechanical properties and effective anti-corrosion properties with resistance to temperature operating service up to 90°C.

5. Three Layer Polypropylene (3LPP) Coating

Fig (4-5) Three Layer Polypropylene (3LPP) Coating

Description:
3-layer coating system composed of FBE primer (first layer), polypropylene based adhesive copolymer (second layer) and polypropylene based topcoat (top layer).
Features:
This coating system combines excellent mechanical properties and resistance to high temperature operating service up to 110°C or 150°C depending on raw materials used.

6. Three Layer Polyethylene (3LPE) Coating

Fig (4-6) Three Layer Polyethylene (3LPE) Coating

Description:
3-layer coating system composed of FBE primer (first layer), polyethylene based adhesive copolymer (second layer) and polyethylene based topcoat (top layer).
Features:
This coating system combines excellent mechanical properties and resistance to temperature operating service up to 60°C (LDPE & MDPE) or 80°C (HDPE) depending on raw materials used.

7. Concrete Weight Coating (CWC)

Fig (4-7) Concrete Weight Coating (CWC)

Description:
Weight coating system composed of cement, water, aggregates, heavy or light depending on the required density, and reinforcement.
Features:
Concrete weight coating is used to provide pipe stability on the sea bed as well as superior mechanical protection. It can be manufactured in a range of densities to suit the project specification.
8. Polyurethane Insulation Coating

Fig (4-8) Polyurethane Insulation Coating

Description:
2-layer coating system composed of FBE and syntactic polyurethane. Polymer or glass microspheres are blended to provide excellent thermal insulation properties.
Features:
Polyurethane insulation systems are designed to cover various water depths. Shallow water products (SPU) are based on a PU matrix into which polymer microspheres are blended to provide excellent thermal insulation properties. Deepwater applications (DWPU) are addressed using a range of products into which glass microspheres are blended. Both products have extensive track records.

9. Polypropylene Insulation Coating

Fig (4-9) Polypropylene Insulation Coating

Description:
Multi-layer coating system composed of polypropylene outer shield, solid polypropylene and foamed polypropylene / syntactic polypropylene.
Features:
Solid polypropylene is used as both anti-corrosion coating and thermal insulation coating where the thermal requirements are not too demanding. Another coating system is side extruded polypropylene foam which is used for thermal insulation of pipelines for water depths up to 600m. Syntactic polypropylene is used to achieve a balance between good thermal performance and thermal insulation capability for deepwater applications.

(b) Alloying the metal
to produce a more corrosion resistant alloy, e.g. stainless steel, in which ordinary steel is alloyed with chromium and nickel. Stainless steel is protected by an invisibly thin, naturally formed film of chromium oxide Cr2O3


4.3.2 Conditioning the Corrosive Environment

(a) Removal of Oxygen

By the removal of oxygen from water systems in the pH range 6.5 - 8.5 one of the components required for corrosion would be absent. The removal of oxygen could be achieved by the use of strong reducing agents e.g. sulphite. However, for open evaporative cooling systems this approach to corrosion prevention is not practical since fresh oxygen from the atmosphere will have continual access.

(b) Corrosion Inhibitors

A corrosion inhibitor is a chemical additive, which, when added to a corrosive aqueous environment, reduces the rate of metal wastage. It can function in one of the following ways:

(i) anodic inhibitors :

as the name implies an anodic inhibitor interferes with the anodic process.

Fe → Fe⁺⁺ + 2e^- eq (4-1)

If an anodic inhibitor is not present at a concentration level sufficient to block off all the anodic sites, localised attack such as pitting corrosion can become a serious problem due to the oxidising nature of the inhibitor which raises the metal potential and encourages the anodic reaction (equation 1). Anodic inhibitors are thus classified as “dangerous inhibitors”. Other examples of anodic inhibitors include orthophosphate, nitrite, ferricyanide and silicates.

(ii) cathodic inhibitors :

the major cathodic reaction in cooling systems is the reduction of oxygen.

½ O₂ + H₂O + 2e → 2OH^- eq (4-2)

There are other cathodic reactions and additives that suppress these reactions called cathodic inhibitors. They function by reducing the available area for the cathodic reaction. This is often achieved by precipitating an insoluble species onto the cathodic sites. Zinc ions are used as cathodic inhibitors because of the precipitation of Zn(OH)₂ at cathodic sites as a consequence of the localised high pH. (See reaction eq(4-2) ). Cathodic inhibitors are classed as safe because they do not cause localised corrosion.

(iii) adsorption type corrosion inhibitors:

many organic inhibitors work by an adsorption mechanism. The resultant film of chemisorbed inhibitor is then responsible for protection either by physically blocking the surface from the corrosion environment or by retarding the electrochemical processes. The main functional groups capable of forming chemisorbed bonds with metal surfaces are amino (NH₂), carboxyl (COOH), and phosphonate (PO₃H₂) although other functional groups or atoms can form co-ordinate bonds with metal surfaces.

(iv) mixed inhibitors :

because of the danger of pitting when using anodic inhibitors alone, it became common practice to incorporate a cathodic inhibitor into formulated performance was obtained by a combination of inhibitors than from the sum of the individual performances. This observation is generally referred to a ‘synergism’ and demonstrates the synergistic action which exists between zinc and chromate ions.

ALSO….. CORROSION INHIBITORS

It is well known in surface chemistry that surface reactions are strongly affected by the presence of foreign molecules. Corrosion processes, being surface reactions, can be controlled by compounds known as inhibitors which adsorb on the reacting metal surface.

The term adsorption refers to molecules attached directly to the surface, normally only one molecular layer thick, and not penetrating into the bulk of the metal itself. The technique of adding inhibitors to the environment of a metal is a well known method of controlling corrosion in many branches of technology. A corrosion inhibitor may act in a number of ways: it may restrict the rate of the anodic process or the cathodic process by simply blocking active sites on the metal surface. Alternatively it may act by increasing the potential of the metal surface so that the metal enters the passivation region where a natural oxide film forms. A further mode of action of some inhibitors is that the inhibiting compound contributes to the formation of a thin layer on the surface which stifles the corrosion process.

4.3.3 Electrochemical Control

Since corrosion is an electrochemical process its progress may be studied by measuring the changes which occur in metal potential with time or with applied electrical currents. Conversely, the rate of corrosion reactions may be controlled by passing anodic or cathodic currents into the metal. If, for example, electrons are passed into the metal and reach the metal/electrolyte interface (a cathodic current) the anodic reaction will be stifled while the cathodic reaction rate increases. This process is called cathodic protection and can only be applied if there is a suitable conducting medium such as earth or water through which a current can flow to the metal to be protected In most soils or natural waters corrosion of steel is prevented if the potential of the metal surface is lowered by 300 or 400 mV. Cathodic protection may be achieved by using a DC power supply (impressed current) or by obtaining electrons from the anodic dissolution of a metal low in the galvanic series such as aluminium, zinc or magnesium (sacrificial anodes). Similar protection is obtained when steel is coated with a layer of zinc. Even at scratches or cut edges where some bare metal is exposed the zinc is able to pass protective current through the thin layer of surface moisture.

In certain chemical environments it is sometimes possible to achieve anodic protection, passing a current which takes electrons out of the metal and raises its potential. Initially this stimulates anodic corrosion, but in favourable circumstances this will be followed by the formation of a protective oxidised passive surface film.

Also …..

Cathodic protection prevents corrosion by converting all of the anodic (active) sites on the metal surface to cathodic (passive) sites by supplying electrical current (or free electrons) from an alternate source.

Usually this takes the form of galvanic anodes which are more active than steel. This practice is also referred to as a sacrificial system, since the galvanic anodes sacrifice themselves to protect the structural steel or pipeline from corrosion. In the case of aluminum anodes, the reaction at the aluminum surface is:

4Al => 4AL⁺⁺⁺ + 12 e^- eq(4-3)

and at the steel surface,

3O₂ + 12e^- + 6H₂0 => 12OH^- eq(4-4)

(Oxygen gas converted to oxygen ions which combine with water to form hydroxyl ions)

As long as the current (free electrons) is arriving at the cathode (steel) faster than oxygen is arriving, no corrosion occurs.

Fig (4-10) Sacrificial Anode CP System in Seawater

· Anodic protection

Fontana and Greene’ state that ‘anodic protection can be classed as one of the most significant advances in the entire history of corrosion science’, but point out that its adoption in corrosion engineering practice is likely to be slow. Anodic protection may be described as a method of reducing the corrosion rate of immersed metals and alloys by controlled anodic polarisation, which induces passivity. Therefore, it can be applied only to those metals and alloys that show passivity when in contact with an appropriate electrolyte. This decrease in corrosion increases the life of components/plant as well as reducing the contamination of the liquid, so is particularly beneficial in the manufacture, storage and transport of chemicals such as acids. Edeleanu first demonstrated the feasibility of anodic protection and also tested it on small-scale stainless-steel boilers used for sulphuric acid solutions .

This was probably the first industrial application, although other experimental work had been carried out elsewhere. Fortunately electrochemical tests in the laboratory can give an accurate assessment of the corrosion behaviour, and the operating parameters for a specific anodic protection system can be obtained .

· Finally the anodic protection is :

• suitable for active-passive alloys (e.g. stainless steel, nickel alloys, titanium)

• requires a broad potential range for passivity

• need sizable/expensive electrical equipment

• risky if potential “slips” into the active/pitting region

• used often for very aggressive solutions when other methods fail, e.g. for protection of tanks storing of strong acids (e.g. sulphuric, phosphoric, nitric)