ANODIC PROTECTION

It was first suggested by Edeleane in 1954. This technique was developed using electrode kinetics principles and is somewhat difficult to describe without introducing advance concepts of electrochemical theory.

Simple anodic protection is based on the formation of a protective film on metal by externally applied anodic current.

Metal 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 potentionstat is an electronic device which maintains a metal at a constant potential with respect to a reference electrode.

Figure.6-8 Anodic protection of a steel storage tank containing sulfuric acid.

The potentiostat has 3 terminals

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.

Table 6 - 4

Anodic Protection Austenitic Stainless Steel at 30  C (Protected at 0.500 volt vs. Saturated Calomel Electrode)

Alloy type 304 (19 Cr- 9Ni) Environment (air exposed) N H 2 SO 4 + 10 -5 M NaCl N H 2 SO 4 + 10 -3 M NaCl N H 2 SO 4 + 10 -1 M NaCl 10N H 2 SO 4 + 10 -5 M NaCl 10N H 2 SO 4 + 10 -3 M NaCl 10N H 2 SO 4 + 10 -1 M NaCl Corrosion rate, mpy Unprotected 14 Anodically protected 0.025

2.9

3.2

1930 0.045

0.20

0.016

1125 0.04

77 0.21

SOURCE : S. J. Acello and N. D. Greens, Corrosion, 18 : 286 (1962).

Table 6-5

Current Requirements for Anodic Protection

Fluid and concentration Temperature



F H 2 SO 4 1 molar 15% 30% 45% 67% 67% 67% 75 75 75 150 75 75 75 93% 75 Metal 316SS 304 304 304 304 316 Carpenter 20 Mild steel Current density, ma/ft 2 To passivate To maintain 2100 390 500 165,000 4700 470 400 260 11 67 22 830 3.6

0.09

0.8

21

SOURCE : C. E. Locke et al., Chem. Eng. Progr., 56 : 50 (1960).

Table 6-5

Current Requirements for Anodic Protection

Fluid and concentration Temperature



F Oleum H 3 PO 4 75% 115% NaOH 20% 75 75 180 75 Metal Mild steel Mild steel 304SS 304SS Current density, ma/ft 2 To passivate To maintain 4400 11 38000 0.03

4400 19000 0.00014

9.4

SOURCE : C. E. Locke et al., Chem. Eng. Progr., 56 : 50 (1960).

COMPARISON OF ANODIC AND CATHODIC PROTECTION

Anodic and cathodic protection tend to complement one another.

Anodic protection - weak-very agressive Cathodic protection -moderately corrosive

It is not practical to cathodically protect metals in very aggresive mediums.

Table 6-6

Comparison of Anodic and Cathodic Protection

Anodic protection Cathodic protection Applicability Metals Active-passive metals only Weak to aggressive All metals Weak to moderate Corrosives Relative cost Installation Operation Throwing power Significance of applied current High Very low Very high Often a direct measure of protected corrosion rate Low Medium to high Low Complex-does not indicate corrosion rate Operating conditions Can be accurately and rapidly determined by Electrochemica Measurements Must usually be Determined by empirical testing

INSTALLATION OPERATING COSTS

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Throwing power or the uniformity of current-density distribution.

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Cathodic protection Low and requires numerous closely spaced electrodes to achieve uniform protection.

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Anodic protection high throwing auxiliary cathode.

power-a single

ANODIC PROTECTION

has 2 advantages.

1. The applied current usually equal to the corrosion rate of the protected system.



not only protects but offers a mean for monitoring instantaneous corrosion rate.

2. Operating conditions can be precisely established by laboratory polarization measurements.

Anodic protection can be classed as one of the most significant advanced in the entire history of corrosion science.

COATINGS

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Metallic and Other Inorganic Coatings

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Relatively thin coatings of metallic and inorganic materials can provide a satisfactory barrier between metal and its environment.

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The chief function of such coating is to provide an effective barrier.

Metal coating are applied by electrodeposition, flame spraying, cladding, hot dipping and vapor deposition.

Inorganics are applied or formed by - spraying diffusion or chemical conversion.

Porosity or other defects can result in accelerated localized attack.

EX. Automobile bumpers, silverware, galvanized steel, tin cans.

bathtubs

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ceramic coatings.

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Electrodeposition –called electroplating consists of immersing a part to be coated in a solution of the metal to be plated and passing a direct current between the part an another electrode.

- Character of the deposite depends on temperature, current density, time, composition of the bath.

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Thick (20 mils), thin (1/1000 mils), dull or bright, soft or hard, ductile or brittle.

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Hard plating – corrosion.

combat erosion

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Single metal, layers of several metals, or even an alloy composition.

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Zinc, nickel, tin, cadmium, gold, silver, platinum.

Ex. Automobile bumper. – inner flash plate of copper, (for good adhesion) – intermediate nickel, (corrosion protection). – top layer chromium (primarily for appearance).

Flame spraying also called metallizing, consist of feeding a metal wire or powder through a melting flame so the metal in finely divided liquid particles is blown on to the surface to be protected.

Oxygen and acetylene or propane – melting flame.

Coatings are usually porous – not protective under severe wet corrosive conditions.

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Porosity decreases with the melting point of the metal. – zinc, tin and lead are better from this standpoint than steel or stainless steel.

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The surface to be sprayed must be roughened (sandblasted).

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Mechanical bond.

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Sometimes paint coating is applied over the sprayed metal to fill the voids and provide a better barrier.

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Porous metal makes a good base for the paint.

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Flame spraying is an economical way.

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Applications include tank cars and vessels of all kinds, bridges, ship hulls and super structures.

Cladding a surface layer of sheet metal usually put on by rolling two sheets of metal together. Example, a nickel and a steel sheet are hot – rolled together to produce a composite sheet with say, 1/8 in of nickel and 1 in of steel.

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High strength aluminum alloys are often clad with a commercially pure aluminum.

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Sometime a thin liner is spot – welded to the walls of a steel tank. Nickel, aluminum, copper, titanium, stainless steels and other materials are often used as cladding for steel.

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Cladding presents a great economic advantage in that the corrosion barrier or expensive material is relatively thin and is backed up by inexpensive steel.

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A high – pressure vessel 1/16 or 1/8 in clad on 3 in of steel.

Hot dipping Hot dip coatings are applied to metals by immersing them in a molten metal bath of low – melting point metals, chiefly zinc, tin, lead and aluminum.

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Thickness of the coating is much greater than electroplates.

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Coated parts can be heat – treated to form an alloy bond between the coating and the substrate.

Vapor Deposition. This is accomplished in a high – vacuum chamber the coating metal is vaporized by heating electrically, and the vapor deposites on the parts to be coated.

This method is more expensive than others and generally limited to “critical” parts. Example, high-strength parts for missiles and rockets.

Deposite aluminum on steel.

Diffusion. Diffusion coatings involve heat treatment to cause alloy formation by diffusion of one metal into the other. Also termed “surface alloying”. Part to be coated are packed in solid materials or exposed to gaseous environments which contain the metal that forms the coating.

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Sherardizing (zinc chromizing (chromium),



coating), calorizing (aluminum) calorizing, the surface is oxidized to form a protective layer of Al 2 O 3 .

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Calorizing permits the use, at high temperature of a strong but easily oxidized steel, such as carbon (with 1% Mo) steel.

Chemical Conversion

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Coatings from chemical conversion are produced by “corroding” the metal surface to form an adherent protective corrosion product.

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Anodizing consists of anodic oxidation in an acid bath to bluid up an oxide layer. – best known – anodized aluminum (Al 2 O 3 ).

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The surface layer is porous and provides good adherence for paints.

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The anodized surface can be “sealed” by exposing to boiling water.

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Bonderizing and Parkerizing (Phosphatizating in a phosphoric acid bath)

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Chromatizing (exposure to chromic acid and dichromates) and oxide or heat coating for steel.

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Glassed Steel or glass-lined steel. – drug industry, in wine, brewery and food plants.

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Concrete is utilized for many corrosion applications. Examples are encasing structural steel, (also for fireproofing), concrete-lined pipe, and concrete vessels.

ORGANIC COATINGS

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These involve a relatively thin barrier between substrate material and the environment. Examples are paints, varnishes, lacquers and similar coatings.

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As a general rule, these coatings should not be used where the environment would rapidly attack the substrate material.

3 main factors to consider.

1. Surface preparation.

2. Selection of primer or priming coat.

3. Selection of top coat or coats

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Surface Preparation – surface roughening, removal of dirt rust, mill scale, oil, grease, welding flux, crayon marks, wax and other impurities. A clean, rough surface is needed.

- grit blast or sandblast.

pickling and other types of chemical treatments.

- solvent degreasing, hot or cold alkali treatments, phophatizing chromate treatment, electrochemical treatment such as anodizing and cathodic cleaning.

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To economic considerations, the selection of surface – preparation method depends upon the metal to be painted, the shape, size and accessibility of the structure; the coating system, and the service conditions

Primers can contain rust-inhibitive pigments such as zinc chromate and zinc dust. Wett ability is needed so that crevices and other surface defects will be filled rather than bridged. Short drying times are advantageous to preclude contamination before the top coats are applied, particularly in field applications.

Top – coat selection is important.

An important point here is that good appearance and good corrosion protection.

Multiple coats are needed so a pinhole in one coat is covered by a complete film of another. Thickness is important also because paint deteriorates or weathers with time.

Various methods are available to reduce maintenance painting costs.

- one is to institute a touch-up program to cover bad spots early.

- apply the paint by a hot-spray method.

- applying tape on edges.

- design the structures such that minimum surface area and edges are presented.

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To sum up, a good paint job consists of proper surface preparation, proper coating selection, and proper application.

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Asphalts and bituminous paints are often used on pipelines.

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Alkyds, glyptols, concrete, red lead, iron oxide, phenolics, lithopones, titanium dioxide paints and chlorinated rubber. Vinyl and epoxy paints have been widely adopted for corrosion applications.

ECONOMICS

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Economic Considerations Control of corrosion is primarily an economic problem. Whether or not to apply a control method is usually determinated by the cost savings involved. The method or methods utilized must be the optimum economic choice.

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Alternative corrosion-control systems vary in cost, and higher cost must be justified.

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With regard to costs of metals and alloys, guideline.

composition is the first

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Type 430 costs more than ordinary steel because of the added 17% Cr,

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Type 304 costs more because of the nickel content.

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Type 316 costs even more because Mo. is an expensive alloying element.

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Copper costs more than iron.

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However, other factors strongly influence the price to the customer.

Ex. Weight bar of steel worth $5, sewing machine needles $5000, wheels for watches $200000.

balance - Tiny electric motor for a missile costs $300, 1-hp. motors $50 - Titanium may be more economical than steel for seawater.

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Corrosion is not a necessary evil. Large savings can be obtained by controlling corrosion. Complete cost and maintenance data are helpful to delineate the high cost items and to determine return on investment. Satisfactory performance and desired life at a minimum total cost per year are all important.