Corrosion occurs when metals come in contact with a corrosive atmosphere. Thus, corrosion can be prevented by: 1. Geometrical Design 2. Proper Selection of Material 3. Modification of corrosive environment 4. Cathodic Protection 5. Anodic Protection.
The most effective and economical method of preventing corrosion is to choose the most suitable geometrical design, most suitable material and then choose most suitable protective method.
Measure # 1. Geometrical Design:
Design should consider mechanical and strength requirements along with an allowance for corrosion. Slight change in design to prevent the corrosion may prove highly economical in many cases. Some simple rules should be kept in mind which designing a part. If these cannot be incorporated in design then, extra protection should be provided to prevent corrosion.
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i. Welded joints be preferred over riveted joints.
ii. Provisions to completely drain off the containers (Fig. 14.40 c).
iii. Avoid using components with residual stresses or under excessive mechanical stress.
iv. Insulate different metals from electrical contacts to prevent galvanic-corrosion by using insulating inserts, Fig. 14.41 or use paint-coat between them. Paint the whole assembly or the nobler metal of the two.
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v. Avoid sharp bends in pipes when high velocities of fluids are involved.
vi. Provide thicker section to take care of impingement attack.
vii. Design properly to avoid excessive vibrations.
viii. Fill the containers with dry-air or inert gas if empty vessels ‘inhale’ moist marine atmosphere.
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ix. Design to exclude the air as oxygen-reduction is the most common cathodic reaction during corrosion. Elimination of O2 prevents corrosion. If metals exhibiting behaviour are involved (such as stainless steel, titanium), then O2 should not be excluded.
x. Avoid heterogeneity, dissimilar metals, vapour-space, stress distribution in a part.
xi. Avoid crevices by filling them by welding or by a filler (Fig. 14.40 a). Also avoid residual moisture. Proper drainage and proper ventilation is important.
xii. Avoid sharp corners. Rounded corners can be given a uniformly thick protective-coat.
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xiii. Avoid contact with absorbent materials.
xiv. Avoid protruding parts.
Measure # 2. Proper Selection of Material:
Though mechanical strength, cost, fabricability, etc. are the main factors in choosing a material, slight variations in composition or structure can help preventing corrosion. Use of purer metals, (devoid of S, P in steel, Fe, Cu, Si in aluminium) improves the resistance but pure metals are expensive, soft and weak. Alloying elements are preferred as a choice to improve corrosion-resistance of metals.
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Some of their functions are:
(a) Oxide-film Forming- Cr, Al in steels; Al in copper.
(b) Oxide-film Improving- Li in Ni; Al in Zn (Hauffe’s rule).
(c) Inhibitor- Arsenic prevents dezincification of brass.
(d) Passivating- Add Cr > 11 % in steels.
Some more factors to be kept in mind before choosing a material for a particular service condition:
i. Metals on anodic end of galvanic series have less corrosion resistance than the metals on cathodic end (more noble). Pure metals have more corrosion resistance than impure metals.
ii. Coarse-grained materials have higher corrosion resistance than fine grained materials.
iii. Cold-worked metal is anodic to annealed metal.
iv. Rough surfaces have lesser corrosion resistance than smooth finished metals.
v. Two-phase material corrodes more than single phase material.
vi. If anodic phase is much lesser than cathodic phase of a two-phase material, pitting corrosion may result.
vii. Isolated metals have better corrosion resistance than a couple as latter easily form galvanic-couple.
viii. Two metals of the galvanic couple if are closer in galvanic series, the corrosion is less.
ix. Change of microstructure improves corrosion resistance. Steels tempered at low or at very high temperatures have better resistance than tempered at 400°C.
x. Avoid having tensile residual-stresses on the surface. Compressive-stresses can be developed by skin rolling, or shot-peening etc.
Measure # 3. Modification of Environment:
Corrosion, very often, can be controlled by changing or modifying the environment.
Some of methods used are:
(A) Elimination of Corrosive Elements:
It can effectively prevent the corrosion.
Some examples are:
(i) Dehumidification of Air:
It is a process of reducing the moisture from air so that the corrosion is almost prevented. When the relative humidity is below the critical value (= below 60%), corrosion is negligible. Freeze-drying can be adopted to do it. The temperature of the room can be increased 6- 7°C above the ambient in the storage area or rooms. Silica-gel or activated alumina can be put along the objects in small closed spaces.
(ii) Removal of Acid:
Removal of acid from water by neutralisation such as by adding lime. Alkaline neutralisers are added to acidic solutions like ammonia, lime, NaOH, sodium salts of petroleum phenols and triethanolamine to have a pH at or above 7. Pourbaix diagram illustrates decrease of corrosion when pH is higher, and the metal becomes passive.
(iii) De-Aeration and De-Activation:
The oxygen-absorption type of corrosion occurs in the presence of O2 at the cathode. Elimination of O2 is essential from the electrolyte to prevent the corrosion.
This is accomplished either by mechanical means (called de-aeration), or by chemical means (de-activation):
(a) Evacuation:
Here the temperature is slightly increased and pressure is reduced with agitation of the electrolyte in a closed container. Dissolved oxygen as well as other gases are expelled from the solution.
(b) The electrolyte can be saturated with an inert gas, such as nitrogen.
II. Chemical Method (De-Activation):
Substances having more affinity for oxygen are added to the electrolyte. For example, sodium sulphite (Na2SO3), or hydrazine (N2H4) are added to water used for steam generation to reduce the corrosion of boilers. Water after treatment is carried through closed-pipes to boilers without coming in contact with oxygen of the atmosphere.
The reactions are:
N2H4 + O2 → N2 + 2H2O …(14.53)
2Na2SO3 + O2 → 2Na2SO4 …(14.54)
(iv) Removal of chloride-ions from electrolyte to prevent pitting and stress-corrosion-cracking of austenitic stainless steels.
(v) Removal of solid particles of the atmosphere by Filtration. Ion- exchangers can be used to remove salts from water.
(B) Inhibitors:
Inhibitor is a substance which when added in small quantity to the corrosive environment decreases the rate of corrosion. Inhibitor either acts as a barrier by forming an adsorbed layer, or retard the cathodic and/or anodic reactions. A cathodic-inhibitor thus increases cathodic polarisation, while an anodic-inhibitor causes anodic polarisation.
A mixed-inhibitor increases both the anodic and the cathodic polarisation. Inhibitors have been developed through empirical experiments. Inhibitors show synergetic (mutually supporting) effect, i.e., the combined inhibiting effect of addition of two or more inhibitors simultaneously is normally more than achieved by addition of one of them.
Inhibitors are often used only in close-systems. Some inhibitors are toxic, and cannot be used where food items are in contact. Inhibitors are often very specific to be used in only certain corrosive environment. Enough quantity of inhibitors should be used to properly protect against the corrosion.
Types of Inhibitors:
i. Chemical Passivators:
Certain substances when added to corrosive environment cause passivity. Such as chromates, nitrates and ferric salts. These substances normally have high equilibrium potential (the redox, or electrode potential) and sufficiently low overvoltage. These are also termed as oxidisers or oxidising inhibitors.
As only complete passivity causes the reduction of corrosion, enough inhibitor should be added to completely passivate the component, otherwise pitting corrosion can occur as often seen at the base of screw-threads.
ii. Absorption Inhibitors:
This is the most commonly used class of inhibitors. These are organic compounds which get absorbed on the entire anodic as well as cathodic surfaces to act as a blanket, and thus effect anodic and cathodic reactions. They are thus also called double-acting (mixed) inhibitors. The absorbed particle need not be an ion, but must have effective electric-dipole.
Absorption inhibitors are commonly added in pickling solutions, and acid solutions used for cleaning clogged steel-water-pipes, boiler tubes. Inhibitor does not allow acid to react with base steel, and thus results in saving of acid, saving of steel and reduction in acid fumes due to hydrogen evolution.
Hexamethylene tetramine and thiourea in amounts up to 0.1% are commonly used such inhibitors. Organic amines such as zinc napthanate, or a compound like lanolin are added in oils, greases or waxes to protect steel temporarily from rusting during storage or shipment.
iii. Film-Forming Inhibitors:
These are very specific inhibitors which stop corrosion by forming a blocking or a barrier film of a material other than the actual inhibiting species itself.
Because of their nature being very specific, film forming inhibitors are classified as:
(a) Cathodic filming inhibitor
(b) Anodic filming inhibitor
(a) Cathodic Filming Inhibitors:
The protective action of the cathodic inhibitor is due to the precipitation of insoluble material produced by the reaction of inhibitor and the alkali produced at the cathodes by such reaction as:
2H+ + 2e̅ = H2
O2 + 2H2O + 4e̅ = 4(OH) ̅
Zinc and calcium salts are commonly used cathodic-inhibitors. Zinc salts produce Zn(OH)2 to protect the metal
Zn2+ + 4(OH) ̅ = Zn(OH)2 …(14.55)
(b) Anodic Filming Inhibitors:
Anodic filming inhibitors polarise the anode to reduce the corrosion current and as well as shift the electrode potential in the cathodic (noble) direction. Benzoate, as anodic inhibitor, inhibits corrosion only in the presence of oxygen.
It behaves similar to absorption inhibitors. Anodic inhibitors should be added in sufficient quantity to cover completely the anode, otherwise rapid corrosion on uncovered regions causes pitting corrosion. Then the damage to the metal is more than without the addition of inhibitor.
iv. Vapour-Phase Inhibitors (VPIs):
Vapour-phase inhibitors are very much similar to organic-absorption type inhibitors. These inhibitors possess a very high vapour pressure, and the vapours of which have corrosion inhibiting properties. These sublime, and then condense on the metal surface. These inhibitors being volatile, spread through the free space, and thus, are used in closed spaces such as inside the packages.
A hydrophobic film (water-repellent) deposits to protect the surface. Sewing needles are wrapped in corrosion-preventive-paper-wraps containing sodium benzoate as chief ingredient. These inhibitors are easy to use, and the components can be used without removing any precipitate from surface. Electronic and precision components are protected by them.
Dicyclohexylamine chromate and benzotiazole are used for copper, phenyl thiourea and cyclohexylamine chromate for brass, Dicyclohexylamine nitrite for ferrous parts.
These inhibitors may help in passivation, or may retard either anodic or cathodic processes.
Measure # 4. Cathodic Protection:
It is a method of reducing or preventing corrosion of a metal by making it a cathode in the electrolytic cell. In free-corroding state, the metal has mixed-corrosion potential, Ecorr, and it corrodes at a rate given by icorr1. If the potential is reduced from Ecorr to E1 by an applied, i1 current from outside, the metal is partly protected as the corrosion current decreases from icorr 1 to icorr 2 If the potential is reduced to Ea (open-circuit potential of anode) by applying an external current i2, the metal shall be fully protected as the corrosion current is decreased to zero.
Ea is the required potential for complete cathodic protection. Ea depends on nature, composition, temperature and pH of the environment. There are two following methods of cathodic protection. In both the methods, a direct-current supply is used. A number of factors control the preference of one method over other.
i. Sacrificial Anode Method:
In this method, another metal, which has more negative electrode potential than the structure to be protected, is connected electrically to the structure, which now acts like a cathode. The structure is protected at the sacrifice (corrosion) of another metal, and that is why, this name is given to the method.
Mg and Mg alloy (6% Al, 3% Zn, 0.2 Mn) are widely used. Zn is also often used. Al and Al-5% Zn alloy are also used. Sacrificial anodes are replaced as soon as consumed. Such a method can be used in remote and difficult-to-reach areas. Cathodic protection is the most effective method, and the only method which gives complete protection from corrosion.
This method, is used for protection of under-water parts of ships, its hull, underground pipes, steel water-tanks, water heaters, condenser-tubes, oil-cargo-ballast tanks etc. Galvanised sheet is basically sacrificial-protection of steel, where Zn is sacrificed.
ii. Impressed-Current Method:
In this method, the given metallic structure is made the cathode by the use of impressed-current by connecting the negative terminal of the external power-supply source to the metallic structure, and the positive is connected to an inert anode of scrap steel, aluminium, graphite, or silicon-iron. Fig. 14.44 illustrates one example. Power supply from mains is transformed, rectified and supplied.
Graphite and silicon-iron are suitable for ground-beds-buried or placed on sea-bed for marine projects. It can supply more voltage and is flexible to be useful to even large objects. Uncoated parts can be protected. It is expensive with common interference problems with parallel currents.
Apart from pipe-lines, underground cables of aluminium, lead; storage tanks, heat-exchangers, steel-gates exposed to sea-water, hulls of ships are protected by this method.
Often stray-current effects are encountered in cathodic protection, i.e., external direct-currents often from other cathodic-protection systems in industrial complexes and densely populated oil-production fields are encountered.
The points where stray-current leaves the structure and enters the soil suffer from accelerated corrosion. Fig. 14.45 illustrates a solution by cooperation between operators. Tank and the pipe are electrically connected, and the placing of anodes rearranged, prevent corrosion of both units.
Impressed current method is being used for high-ways and other bridges. Solar-panels are being used to supply current for bridges in rural areas to prevent corrosion of reinforcing-steel.
Measure # 5. Anodic Protection:
If active-passive metals, particularly nickel, iron, chromium, titanium and their alloys (even alloys having these metals), are supplied a carefully controlled anodic current, they get passivated as protective film forms on anode material, the rate of corrosion decreases, or the corrosion is prevented.
Potential more than flade-potential brings the metal in the passive range to decrease the corrosion-current drastically, when little or no corrosion takes place. A potentiostate is used to maintain the metal at a constant potential with respect to a reference electrode.
Metal must exhibit passivity in the corrodent. Anodic protection reduces capital investment, maintenance, hydrogen-embrittlement, equipment cost and contamination. Steels and stainless steels have been protected in extremely corrosive conditions.
