CORROSION CAUSES & ITS INHIBITATION

S. L. PANDITLECTURER/EM

Corrosion of centre sill

INTRODUCTION • Corrosion: an inevitable phenomenon• Corrosion: a major problem faced by IR. • Most corrosive sites: Industrial or marine atmosphere

[presence of appreciable amounts of SO2 (burning of coal, gas & gasoline),which converts to H2SO4 & lesser amt of H2S, NH3, NO2]

• Least corrosive atmosphere: Desert sites• In the corrosion process, the metal looses its:

– Properties– Appearance

• Corrosion results in loss of section affecting safety and high cost of repair in maintenance.

CAUSES OF CORROSION• Pure metals exist in combined (natural) state.• They get reverted back to combined forms (corrode)

when come in contact with:– air– aqueous solution [(moisture in the form of rain, dew,

condensation, high RH), neutral water, man made soln.)]– acids – bases – salts (may destroy any protective oxide layer acquired by

metals)– gases, etc

• Corrosion is more severe in areas: – where there is accumulation of dirt, dust (may contain

chlorides), sand or other contaminants.

IMPACT OF CORROSION ON ECONOMY• Iron & Steel: the commonest metal in use.• The quantity of iron & steel destroyed every

year, equals to about a quarter of its annual world production.

• The approx estimate annual loss of metals is 2- 2.5 billion dollars all over the world

• In India, it is about Rs.500-600 crores per annum

• Our economy will be directly changed if there were no corrosion.

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CORROSION :DEFINITION

Definition:“Corrosion is the slow but spontaneous deterioration of materials by chemical or electro-chemical reaction with its environment”.Example- –Formation of a layer of reddish brown scale

on the surface of iron (rusting of iron)–Formation of green layer on the surface of

copper when exposed to moist air and CO2

CORROSION :DEFINITION

Corroded steel bolt (Reddish brown scale of Fe2O3.nH2O)

Exposed copper plate (Green layer of basic carbonate)

Green deposit on the surface of the corroded impellor

THE CORROSION PROCESSFour factors must be present for corrosion to take place:-

• Metal• Oxygen• Temperature

– the rate of corrosion increases with rise in temp.– below zero degree corrosion ceases

• Moisture:– essential for ionic reaction– may/may not be present as a liquid, but

corrosion can proceed if RH is 60% or above.– corrosion practically ceases below 60%RH

CORROSION RATE• Corrosion rate is aggravated by:-

– Surface contaminants:• mill scale, surface rust, dust, etc

– Defects:• scratches, machine marks, pits, dents, ridges

from local stresses, porosity and casting defects.

CORROSION RATE EXPRESSIONCorrosion rate is expressed in terms of loss of thickness:

mpy=534 W/DATwhere, mpy=mils per year (1 mil= 0.025”)W=weight loss (mg)D=density of specimen (g/cc)A=area of specimen (square inch)T=exposure time (hrs)

TYPES OF CORROSION• Dry or chemical corrosion• Wet or electrochemical corrosion

Dry or chemical corrosion: “Direct chemical action of metals with environment/atmospheric gases [O2,H2,H2S (generated naturally by the decomposition of organic S-compds in polluted rivers), SO2, N2, Halogens, etc] or anhydrous inorganic liquid or gases (Cl2, F2, CO2, etc)”.Wet or electrochemical corrosion:“The corrosion involving the flow of electrons from anodic area towards cathodic area through a conducting solution ”.

MOST IMPORTANT FORMS OF CORROSION

• Uniform corrosion/Atmospheric corrosion/General corrosion

• Galvanic cell corrosion• Crevice corrosion• Pitting corrosion• Stress corrosion• Erosion corrosion• Fretting corrosion• Intergranular corrosion• Poultice corrosion• Filiform corrosion • Selective leaching

CAUSES OF CORROSION

CAUSES OF CORROSION ON RAILWAY ROLLING STOCK

Indian Railway is the third largest transport network in the world comprising a fleet of rolling stock item like:

Coaches – 40,000 Wagons – 4,00,000 Locomotives – 10,000 They are exposed to all weather conditions like

variation in day & night temperature, humidity, salinity, rainfall, etc apart from industrial/marine atmosphere which makes them susceptible to corrosion.

CAUSES OF CORROSION (CHG)

CAUSE OF CORROSION: COACHES1. Water seepage through flooring to the top of the

trough floor2. Leakage of water through lavatory flooring.3. Habits of flushing the flooring with the water jet for

cleaning.4. Defective water pipe fittings5. Surface preparation not prepared properly for

carrying out welding activity.6. Surface preparation not prepared properly before

starting of painting7. Blockage of drain water by accumulated dust :

– On the top of trough floor– On the top of plate in Inner Head stock.

AREAS CORRODED IN PASSENGER COACHES • Trough floor: areas adjacent to lavatory, doorways,

luggage compartment in SLR (sleeper) & kitchen area of pantry car.

• Head stock: areas inner & outer head stock, stiffener, protection tube area, support plate area etc.

• Side wall bottom: area below lavatory & between body side pillars

• Body pillars: bottom portion of pillar near doorways & lavatory area

• Cross bearers: joint between sole bar & cross bearers.• Body side door: bottom of door due to accumulation

of dust• Battery boxes: corrosion due to acid action• Roof: ventilator & area around it.

CORROSION IN COACHES

Wash basin drain pipe Window sill

CORROSION IN COACHES

Heavy corrosion over window sill Heavy corrosion on roof

RCF Coach

CORROSION IN COACHES

Heavy Corrosion Raised Trough floor

Rajdhani Exp. Coaches

CORROSION OF WAGONS

FACTORS RESPONSIBLE FOR CORROSION OF WAGONS

• Type of commodities loaded• Running through coastal areas• Contamination of rain water mixed with

chemical substances of the loaded commodities

• Seeping in of rain water inside the wagons• Painting without surface preparation• Water logging in crevices & overlaps• Flowing of deposited rain water from floor to

the sole bar near doorways

MAJOR LOCATIONS OF CORROSION OF WAGONS

• Side panel• End panel• Head stock• Body pillar/stanchion• The sole bar at/near doorways• Portion of the side panel at the ends near the

corner angle• Floor• Interior surface of the wagons.• Under frame components (cross bar, longitudinal

bar, centre sills, etc).

CORROSION PRONE AREA (BOXN)

• Side panel• Overlapping joint

Reasons for corrosion1.Contaminated water mixed with chemical substances of loaded Materials.2. Running through coastal areas. 3. Deposition of rain water.

CORROSION PRONE AREA

• End panels adjoining with floor plates

Deposition of rain water mixed with the chemicalsubstances of loaded commodities.

CORROSION PRONE AREA

• Sole bar• Door Hinge• Floor

Corrosion causes:-1. Due to flowing of rain water from floors near door way.2. Door hinges get corroded in contact with flowing rain water from floor.3. Floor gets corroded due to deposition of rain water and water mixed with chemical substances contaminated with chemical substances of loaded materials.

CORROSION PRONE AREA (BOXN)

• Joint area between Door way stiffener with side stanchion

CORROSION INHIBITATION

METHODS OF CORROSION INHIBITITION

1. Proper designing2. Proper selection of materials for the

environment concerned3. Cathodic protection4. Modification of the environment5. Corrosion inhibitors6. Protective coatings

PROPER DESIGNING &

MATERIAL SELECTION*selecting the proper material is essential for long term corrosion

control

DESIGN & MATERIAL SELECTION PRINCIPLE

• Avoid galvanic corrosion by following methods:-– Never join different metals & alloys, if not possible,

insulate them with efficient moisture resistance electrical insulator.• Greater the separation, more rapidly anode will

corrode– Never use a design, if relative sizes of the cathode

& anode are widely different. • (area of anode/area of cathode>1)

DESIGN & MATERIAL SELECTION PRINCIPLE– Manufacture bolts, screws, nuts & welds from a

more noble materials than the members to be joined

– No problem occurs, if the area of the anode is large compared to the cathode

– Continuous welds should be preferred, not intermittent (responsible for crevice corrosion)

Discontinuous welding

DESIGN & MATERIAL SELECTION PRINCIPLE• Avoid L, T & U shaped profiles in constructions:

– greater the no. of angles, corners & edges, it becomes more difficult for efficient surface treatment.

• Avoid improper insulation:– leakage of current can take place which causes

anodic corrosion.• Avoid the contact of metallic object with moisture:

– Design should be such that retention of moisture is as low as possible

– Design the storage containers should be such that they can be completely drained & cleaned.

DESIGN & MATERIAL SELECTION PRINCIPLE

Water storage tank Water storage tank

DESIGN & MATERIAL SELECTION PRINCIPLE• Using of pure metal:-

– Purity increases the CR• Use of metal alloys:-

– Suitable alloying increases corrosion resistance. • Elimination of tensile stress:-

– Internal stresses & corrosion can be reduced by heat treatment

ICF BMP DESIGNS

Trough floor - 1.7mm thick to Specn. AISI 301 with 2D finish

Body side Pillar - 2mm thick to Specn.AISI 301

SS Brake Pipes & Fittings SS Lavatory Inlay - 2mm thick to Specn. AISI 304 with 2D finish

DESIGN & MATERIAL SELECTION PRINCIPLE

DESIGN & MATERIAL SELECTION PRINCIPLESS Lavatory Inlay - Western Style SS Bottle Holder

SS Ceiling Fan SS Seat Frames

DESIGN & MATERIAL SELECTION PRINCIPLEA View Of Kitchen Area Service Counter

FRP Body Side Door SS Shield For Window

DESIGN & MATERIAL SELECTION PRINCIPLE

Appearance Of Stainless Steel Body Structure

SS Body Structure

Pantry Car Side wall, roof and partition in the kitchen area to Specn. AISI 304

CATHODIC PROTECTIONDefinition:- ‘’Cathodic protection is defined as the control of electrolytic corrosion by forcing the structure to be protected to behave like a cathode”.Methods:1.Sacrificial anode method2. Impressed current method

• The method involves the use of a more reactive metal as sacrificial anode along with the less reactive metal to be protected.

• Zn, Al, Mg & their alloys are used as sacrificial anodes

MODIFICATION OF ENVIRONMENTDeaeration: Oxygen/dissolved oxygen from water is removed by physical or chemical means & accomplished through the use of oxygen scavengers [sodium sulphite & hydrazine(N2H4)]

Dehumidification: Moisture from air is removed by dehumidification using silica gel

Neutralisation of acids: Corrosive environment due to presence of acids(HCl, SO2, CO2, H2S, etc) can be minimised by injecting alkaline neutralisers (NH3, NaOH, Lime, etc)

CORROSION INHIBITORS• Some inorganic salts like metal silicate &

borates are added in small quantity

PROTECTIVE COATINGS• Coating of grease can be used to give a

temporary protective coating• Organic polymers or rubber latex can be

applied to give coatings which can be stripped off when required

• Paint: The most common coating

CLASSIFICATION OF PROTECTIVE COATING

Types of coating applied on the metal surface:-

• Metallic coatings • Chemical conversion coating• Organic coating

METALLIC COATINGS• The metallic coatings used are:-

– Zn, Sn, Ni, Cr, Al, etc • Coating separates the base metal from the

corrosive environment • Acts as an effective barrier

METALLIC COATINGS• Hot dipping:

– Galvanization– Tinning– Aluminizing

• Cementation or diffusion coating:– Chromizing (coating of steel with Cr)– Sheradizing (coating of steel with Zn- used in Bridge girder)– Calorizing (coating of steel with Al-used in Ganga bridge)

• Electroplating:• Chromium plating• Nickel plating• Copper plating

• Cladding: (not used in Rlys)

GALVANISING

“Thin coating of Zn- layer on iron & steel sheets”

• Protects iron/steel even when the coating is scratched or porous

• only effective for a limited time since Zn dissolves quickly once electrolytic reaction begins

• Galvanised steel resists corrosion far longer than untreated steel

• galvanising is better than tinning

TINNING

“Coating of tin on ferrous or non-ferrous articles” *Does not protect iron/steel when the coat is scratched or porous because of iron is anodic to Sn

CEMENTATION• Base metal (steel) is heated with fine

powdered coating metal (Zn, Cr & Al)The process is called:– Sheradising, if Zn is used; – Chromising, if Cr is used; – Calorising or Alonising, if Al is used.

• Very good bond is produced• The process is limited to relatively small objects

ELECTROPLATING• The freshly cleaned metal is made cathode in a

suitable electrolyte bath containing:-• Extra reagents to increase conductivity & to aid

the formation of smooth, dense & coherent coating

• Different electroplating:-– Chromium plating– Nickel plating– Copper plating

METAL CLADDING“The process by which a dense, homogeneous layer of coating metal is bonded permanently to the base metal on one or both sides”

• It represents a great economic advantage in that the corrosion barrier or expensive material is relatively thin

Cladding of Duralamin

CHEMICAL CONVERSION COATING

Chemical conversion coating

Anodising Phosphating (Bonderising) Anodising: Al develops an oxide surface layer in air which offers some CR. This

can be improved by thickening the by an electrolytic process. This treatment is known as Anodising. The freshly formed anodic layer is porous & has to be sealed by immersion in boiling water to give max protection against atmospheric corrosion. This anodic layer increases surface hardness, abrasion resistance & CR.

Phosphating : In this process, steel surfaces are treated with phosphoric acid or solutions

containing phosphate ions The treatment results in the removal of surface rust & the steel surface is

converted to metallic phosphate (protective). The coatings do not provide appreciable corrosion protection. They are useful mainly as a base for paints, ensuring good adherence of paint to

steel.

PHOSPHATING• Different steps involved

in hot zinc phosphating process:-– Alkali degreasing– Cold swilling– De – rusting– Cold swilling– Hot phosphating– Cold swilling– Sealing/Passivation

ORGANIC COATINGS

Organic coatings

Liquid coating Powder coating

DIFFERENT LIQUID PAINTS USED IN IR

ALKYD PU

EPOXY PUPack ‘A’- Base Pack ‘B’- Hardener

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LIQUID PAINTALKYD PAINT: A single pack system.

• Produced by the polymerisation of Polybasic acids with polyhydric alcohols in correct proportion in presence of heat and catalyst (CO2).Phthalic Acid + Glycerol = Alkyd Resin (thermoplastic)

PU PAINT: A two pack system. • Formed through a reaction of Aliphatic hexa methale

isocyanate with polyol .Poly isocyanate + Polyol = PU Resin

EPOXY PAINT: A two pack system.

• Made from Epichlorohydrin & Bisphenol ‘A’ in excess of Epichlorohydrin . Epichlorohydrin + Bisphenol “A” = Epoxy Resin

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LHB RAJDHANI PAINTED SHELL

POWDER COATING

POWDER COATING GUN POWDER COATING BEING DONE

PROCEDURE: ELECTROSTATIC SPRAYING• Positive electrical charge is applied to the dry powder

while the work piece is electrically grounded.• The charged powder & grounded work piece create

an electrostatic field that pulls the paint particles to the work piece.

• The coating deposited on the work piece retains its charge, which holds the powder to the work piece.

• The powder coated work piece is placed in a curing oven, where the paint particles are melted on to the surface & the charge is dissipated.

COMPONENTS READY FOR DESPATCH

POWDER COATING OF RAILWAY COACHING COMPONENTS

• Side wall panel• Roof panel• Partition frame• Chair pillar• Kitchen berth frame• Push plate• Spring cover plate• Basin cover• Soap dish• Flushing valve

• Curtain rod• Bottle opener• Guard rail bearing• Tissue paper box• Shutter• T- bolt assembly• Towel rods• Long hinge. etc

Done at Liluah workshop:------

THANK YOU