CORROSION AND PROTECTION

COATING

Anodizing        Metallic coating       OrganicCoating        InorganicCoating            Nano Coating 

Electroplating   ElectrolessCoating  HotDipCoating MetalClading ThermalSpray  PackCementation   Chromating  Phosphating    Enameling

Anodizing

An oxide film can be grown on certain metals – aluminum, niobium, tantalum, titanium, tungsten, zirconium – by an electrochemical process called, anodizing. It will protect the metal parts by making the surface much harder than natural aluminum.  Aluminum oxide is grown out of the surface during anodizing and then becomes aluminum hydrate that is extremely hard.  The porous nature of the anodized layer allows the product to be dyed any color that is required.  Anodized surfaces can typically only be dyed black or dark green due to the denser pore size.

Procedure

In an anodizing cell, the aluminum workpiece is made the anode by connecting it to the positive terminal of a dc power supply. The cathode is connected to the negative terminal of the supply. The cathode is a plate or rod of carbon  lead, nickel, stainless steel – any electronic conductor that is unreactive (inert) in the anodizing bath. When the circuit is closed, electrons are withdrawn from the metal at the positive terminal, allowing ions at the metal surface to react with water to form an oxide layer on the metal. The electrons return to the bath at the cathode where they react with hydrogen ions to make hydrogen gas.

 

 

 

 

 

 

 

Anodization of aluminum is a process of following electrochemical reaction

Anodic dissolution at the metal | Oxide Interface           2Al  =  2Al3+  + 6e                           (1)

At    Oxide| Electrolyte Interface                                   3Al + 3O2- =Al2O3  +  6e                  (2)

Cathodic   reaction                                                       12H+   +  12e= 6H2                              (3)

Pores are created by the reaction (1), while growth of oxide occurs due to reaction (2). By controlling the rates of these two reactions ,. Percentage porosity as well as pore size  can be retrained.   Al3+ ions are generated and accumulated at the  metal | Oxide Interface,  while  O2-  ions from the solution specifically adsorb at the  Oxide| electrolyte Interface , giving rise to layer of opposite charges across the two faces of the oxide layer, producing an electrical field. Application of external potential ( anodization voltage ) control the field and mobilization of oppositely charged ions towards the opposite direction, causing porous anodized oxide. Since under a minimum electrical field strength , ions initiate to move ,  a minimum potential  is to produce the porous anodization.

DC anodising in electrolytes of sulphuric acid uses a 15-20 % sulphuric acid solution. The current applied is direct current. At voltages of 12-20 volts, current densities of 1-2 A/dm2 are applied. The bath temperature is between 18 C and 21 C. DC anodising in electrolytes of sulphuric acid/oxalic acid differs from the abovementioned method in that in addition to the 15-20 % sulphuric acid, the electrolyte solution contains approximately 10 % oxalic acid. This allows anodising at slightly higher temperatures, between 20 C and 25 C. This, however, requires a somewhat higher voltage of 20-25 volts. The applied current density is also 1-2 A/dm2.

 

 

Nanostructure materials by Anodization

Nanostructured materials can be produced by various methods ,viz. Sol gel technique, mechanical ball milling, lithographic method etc. The template method entails synthesizing desired material within the pores of nanoporous membrane. In this case the size and shape of the nano-structures are controlled by the size and shape of the template. The template method has number of interesting and useful feature. This method can be used to produce conductive polymers, metals, semiconductors, carbon tube. Furthermore nanostructures with extraordinary small diameter can be prepared and the diameter of the hole can easily controlled by external conditions  such as acid concentrations and applied potential.

Pure aluminum was anodized in oxalic , sulfuric and chromic acids in order to form nanoporous alumina template for fabrication of Nanowire or nano rod. Porous anodized film was successfully produced under controlled process parameters. Pore diameter varied with type of acids , concentration and potential. The sample was anodized in  galvanostatic circuit using Al cathode. Initially the concentration of acids,  oxalic acid, sulfuric acid and chromic acid, was fixed at 15% and temperature at 25 C. Of the three acids used anodization in chromic acids produced narrower pores. Further experimentations were conducted varying the concentration in 10% ,20% and 30% chromic acid. Pores sizes in the order of  50-200 nm are found

 

 

 

 

 

 

 

 

 

 

 

Metallic Coating

 

Extensively used for protection ,   Thickness range – from <1μm to >5mm.

 Two classes – Noble (barrier) and Active (both barrier & sacrificial)

Can be applied to almost all other metal by the following methods:


• Electroplating  • Electroless plating   • Hot dip coating   • Thermal spraying   • Cladding 
• Pack cementation  • Vapour deposition  • Ion implementation  • Laser processing


Electrodeposition

 

Uses: decoration; resistance to corrosion, wear, chemicals; electrical properties;magnetic properties; solderability; reflectance; reduction of friction; etc.

Either a barrier protection or both barrier & sacrificial actions ,


Electrodeposition of an adherent metallic layer on substrate used as cathode with an inert anode and source of metals are in the form of ions in the electrolyte. Control of potential, current density, concentration, pH and temperature produce

desired properties of the coating.

Examples: - Sn or Cr plating on steel sheets for food packaging & container uses
- EG or Zn plating of steel sheets, wires, stampings, forgings, screw
- Zn or Cd plating of fasteners & hardware items
- Bright Ni-Cr plating for household appliances & interior auto-hardware

Electroless  Plating

 

Autocatalytic chemical reduction of Ni-ions from an aqueous solution; No electrical current required

 

NiSO4 + 3 NaH2PO2 + 3 H2O → Ni + 3 NaH2PO3 + 2 H2↑ + H2SO4
3 NaH2PO2 → NaH2PO3 + 2 P + 2 NaOH + H2O

Two types: Ni-P & Ni-B alloy coatings - for corrosion & wear resistance

 

Ni-P: - either low P (1-4 %) or medium P (5-8 %) or high P (9-12 %)
- corrosion resistance in neutral to acidic media with P content

 

Ni-B: - typically 5% B
- excellent resistance to wear & abrasion but less resistance to corrosion
- more expensive than Ni-P

Hot Dip Coatings

Coatings by immersing in a liquid metal/alloy bath;  Metallurgically bonded to the substrate

Advantages: - articles with intricate design can be easily coated
- minimum coating thickness can be maintained easily
- resistance to mechanical damages
- good resistance to corrosion
- good adhesion due to metallurgical bonding

• Limitations: - coating metal must melt at a reasonably low temperature
- base metal should not undergo any property changes

• Galvanised -- Zinc (ASTM A653)
Wide variety of uses
• Galvannealed -- Zinc + ~10% iron (ASTM A653)
Intended to be painted; automobile & white goods application
• Galvalume -- 55% Aluminum/43.5% Zinc/1.5% Silicon (ASTM A792)
Main use is metal roofing & siding
• Galfan -- Zinc + 5% Aluminium (ASTM A875)
Used for deep drawn parts & prepainted siding
• Aluminised -- 2 types – pure aluminum coating & aluminum + 5 to 11%
silicon coating(ASTM A463)
Heat-oxidation resistance is excellent for the Si- containing coating
• Terne -- Lead + 3 to 15% tin. (ASTM A308)

Thermal Spray Coatings

• Finely divided molten metallic/nonmetallic materials, originally in powder/wire form
are sprayed onto a prepared metal surface
• Fed through a gun, assisted by a stream of compressed gas

On striking the metal surface droplets flatten into platelets & adhere to the surface,
particle by particle, to form a thick lamellar coating (50-500 μm)
• Heat source: combustion of gases (e.g. acetylene), electric arc or plasma

• Deposited material may not be in a pure form – may react with the process gas or
surrounding atmosphere
• Most commonly used materials: Al & Zn; also stainless steel, Ni-base alloys, ceramics
• Common uses: TV towers, antennae, radar, oil rigs, ships, bridges, light poles, girders
fuel storage tanks, high temp. kilns, etc.


Metal Cladding

 

A thin sheet of metal is bonded on the base metal
• Intimate contact force: hot or cold rolling, hot pressing, explosive bonding or weld
cladding

In different forms: plate, sheet, tube, rod, wire
• Example: Cu, Ni-alloys, Ti on steel, wide
combination
• Different Clad systems:
- Noble metal (SS on Al)
- Corrosion barrier
- Sacrificial metals (Alclad – more
active Al alloy on noble Al alloy)
- Transition metal (battery: Cu-SS-Ni)
• Common use: pressure vessels, reactors, heat
exchangers, storage tanks, jewelry, electrical &
electronics applications

 

Pack Cementation (Aluminising)


• Al diffuses through steel surface and develop aluminide intermetallic coatings
- superior resistance to oxidation, carburisation & sulfidation
- performance of steel in high-temp. corrosive environment
• Common uses: - hydrocarbon processing (heater tubes, furnace tubes)
- industrial furnace components
- power plants (boiler tubes, combustor tubes etc.)
- chemical processing (reactor vessels & tubing)
• Aluminising process: heated in a pack of Al-powder, an inert filler (Al-oxide) and an
activator (halide salt) under an inert atmosphere in a retort
• Activator reacts with Al to form volatile Al-halides that react with steel to deposit Al

Organic Coating

Components:  Binder; Pigments; Solvent & Other Additives

i) Binder:
• Continuous phase in a paint film
• Largely responsible for the protective and mechanical properties of paint
• Generally organic materials such as oleoresinous varnishes, resins
(alkyds, epoxy esters, urethane oils), treated natural products (cellulose
nitrate, chlorinated rubber), and completely synthetic polymers.
• Few inorganic binders - pre-hydrolysed ethyl silicates, alkali silicates etc.

ii) Pigments: 

• Finely divided solids of 0.2-10 μm size
• Added for various functions e.g. developing colour and obliteration or
hiding power, inhibiting rust formation, protecting from sunlight and
weather and reinforcing the film

True pigments are used to provide colour and opacity or hiding power
• A pigmented film is more weather resistant than an unpigmented film
of the same binder
• Specific pigments are used in the primer for metals to inhibit corrosion
• Extenders: practically transparent; no contribution to colour or opacity
• used in certain types of paints (primers, undercoats and some low
gloss finishes) to control the physical properties
• Natural pigments: Fe-oxides & hydroxides + clay or siliceous matter
• less bright colours than the corresponding manufactured ones

• Manufactured inorganic pigments: whites and a wide range of colours;
carbon black (elementary carbon), Zn-phosphate, Zn-oxide, Fe-oxide,
calcium carbonate, Zn and Pb chromate
• Organic pigments: entire spectrum range, brighter than inorganic ones,
but the brilliance and opacity vary considerably
• Pigments from plants and animals lack permanence, no longer in use

 

iii) Solvent: 

 

• Usually volatile organic liquids that dissolve the binder
• Flow and gloss control agents, dryer, anti skinning and anti-settling
agents, and surface active agents used to assist pigment dispersion.
• Act as means of conveying pigment-binder mixture to metal surface
• Do not take part in film formation
• Solvent should evaporate completely after paint application
• Common solvents: various hydrocarbons (toluene, xylene, trimethyl
benzene, white spirit, etc.) alcohols, esters, ketones, etc.
• Water is also used as solvent in water-borne and emulsion paints
• Flammable and possess a degree of toxicity - use of organic solvent
based system is becoming limited due to stringent regulations

 

iv) Other miscellaneous additives:


• Flow and gloss control agents, dryer, anti skinning and anti-settling
agents, and surface active agents used to assist pigment dispersion.

Inorganic Coatings

 

 Enamel Coatings

 

• Applied on fabricated sheet steel, cast iron or aluminium parts
• Makes decorative and corrosion resistant surface
• Thickness : 50-100μm
• Vitreous nature
• Fuse with the substrate after their application and firing at 425C or above
• Offer only barrier protection to the substrate
• Coatings must be free from defects and coating discontinuities
• Brittle and easily damaged by impact & thermal shock and difficult to repair
• Common uses: major appliances, sanitary ware, water heater tanks,
chemical reactors, heat exchangers, barbecue grills and other cookware,
architectural panels and outdoor signs


Conversion Coatings

Chemical or electrochemical treatment of metals
• Complex, adherent and protective coating compounds comprising the
substrate metal and components of the processing bath are formed on
the metal surface
• 2 common types: phosphating & chromating

Phosphating

• Dilute solution of phosphoric acid and other chemicals (Fe/Zn/Mn phosphate)
• A gray to black integral layer of insoluble metal phosphate forms on the metal
surface
• Iron, steel, galvanised steel or aluminium can be treated
• 2 methods of applying phosphate conversion coatings – spraying method (less
expensive), and immersion coating (for better quality)

• Uses: base for paints (automobile bodies, appliance cabinets etc.); corrosion and
wear resistance; lubricity in cold forming and electrical insulation
• Fe phosphates – thinnest, least protective, amorphous ()
Good base layer for paints, least expensive ()
Can be used at highest acidity level to remove rust, avoiding a pickling step ()
• Zn phosphates & Mn phosphates – crystalline, can hold oils and waxes very well to
improve corrosion resistance (when both phosphate and oil/wax are used) ()
However, layers are generally too thick to be used in coil coating where painted
sheets are further formed ()
• Heavy Mn phosphates – not painted; porous enough to hold corrosion inhibitors
along with the oil or wax; usually applied to steel nuts, bolts and screws

Chromating


• A chemical or an electrochemical treatment to form hydrated chromate coatings
• Generally 10-1000 nm thick, amorphous, and contain other metals and anions
• Colour: transparent to bluish and greenish and to finally yellow for thicker coatings
• Method: usually by immersion or spray
• Steel, aluminium, zinc, magnesium, cadmium, copper etc. can be treated
• corrosion resistance (bare or coated), adhesion of paint or other organic finishes,
and for decorative finish
• Disposal of Cr treated waste – hazardous (Cr+6 is toxic)
• Use of Cr+6 in chromating for automotive steels – banned in USA & UK
• Cr+3 & Cr-free formulations already available in market