4 types of oxide layers: -

1. Porous oxide layer (non – protective): -

• If the oxide layer is porous then through pores – penetration of o2 gas taking place and metal get corroded.

e.g.: - fe,Mg,Na,K,Ca, etc.

reaction: - 2fe + o2 – 2feO

                     2Mg + o2 – 2MgO

2.     Non – porous oxide layer (protective): -

• If the oxide layer is stable non – porous strong then further corrosion of metal will be stopped.

e.g.: - chromium,nickel, Sn (tin),lead,aluminium,zinc,bronze,various metalalloys .

reaction: - 1. 2AL + O2 – AL2O3

3.     Unstable oxide layer: -

• When oxide layer is unstable, and it decomposes back into metal and o2 gas then there is no corrosion of metal taking place.

e.g: -Ag,AU, Pt.

4.     Volatile oxide layer: -

• If the oxide layer is volatile, then its evaporation taking place the under lying metal gets exposed for further attack of o2 gas and corrosion taking place.

• e.g: - MO2O3 (molybdenumoxide)

dillingbedworth rule / ratio (PBR)

• This rule gives an idea about nature of oxide film, whether it is porous or nonporous.

• The rule states that,Metal ‘if the volume of oxide < volume of metal consumed

Then,

       Oxide layer – porous

Else, oxide layer – nonporous

PBR =

If, PBR > 1 but PBR < 1.45

Metal oxide layer – nonporous(protective)

B] Mechanism of dry corrosion due to other gases: -

a)     Dry corrosion due to chlorine: -

Reaction: - 1. 2Ag + Cl2 – 2AgCl

Agcl forms thin protective (non-porous) layer on the metal surface.

             Non corrosion taking place

Sn + 2cl2 – SnCl4

 SnCl4 forms thin, non-protective layer on the metal surface

                       Corrosion taking place

b)    Due to So2: -

• Sulphur oxide is produced due to combustion of fossil fuels. when So2 attacks on metallic surface in the form of H2So4, corrosion of metal taking place.

Reaction: - SO2 + H2O – H2SO3

                     SO2 + H2O ½ O2 – H2SO4

c)     Due to oxides of nitrogen: -

• Oxides of nitrogen are produced due to combustion of fuels it forms HNO3 which cause corrosion of metal.

Reaction: - NO2 + O3 – NO3 + O2

                      NO 2 + NO3 – N2O5

                       N2O5 + H2O – 2HNO3

d)    Decarburisation: -

• The processes of removal of carbon of carbide from the metal by action of hydrogen at higher temperature is called as decarburisation.

e)     H2 embrittlement: -

• Breaking of metal at the weekend region by hydrogen accumulation in metal called as H2 embrittlement.

f)       Presence of co2: -

• Due to co2 formation of h2co3 taking place and it reacts with metal to occur corrosion.

H2O + co2 – H2CO3

Fe + CO2 – FeO + CO

EMF series and galvanic series

2)  wet corrosion: -

• The corrosion occurs due to aqueous conducting medium with the formation of anodic and cathodic areas.

2types: -

A)    Galvanic cell wet corrosion

B)    Conc. cell wet corrosion / differential wet corrosion

Galvanic cell wet corrosion: -

• It occurs due to aqueous conducting medium with the formation of galvanic cell.

Conc. cell wet corrosion: -

• It occurs due to aqueous conducting medium with the formation of conc. Cell due to variation of oxygen conc. Metal.

Possibilities: -

• Two dissimilar metal
• Stressed and unstressed part
• Presence of impurities in the metal

Possibilities: -

• Partial part of metal dipped in water.
• Partially buried part of metal in ground.
• Crevices
• Partially oxide layer on metal surface

Mechanism of wet corrosion: -

1. Reaction on anode: -

• Metal atoms on the surface of anode pass into conducting medium by forming metal ions, leaving behind electrons.

M – m+n + ne-

2.     Reaction on cathode: -

• The electrons left on anodic part, flow to the cathodic part depending upon the nature of conducting medium they bring about one of the following reactions: -

a)     H2 liberation (H2 Evolution): -

• If the corroding medium is acidic then H + ions from the medium capture the electrons from the cathode and there is liberation of H2 gas taking place.

b)    O2 absorption: -

• If the corroding medium is neutral or slightly alkaline containing some oxygen gas dissolved the reaction of o2 absorption takes place on the cathode.

C)   Plating: -

• If the cathodic part is in the contact with ions of lower plotted metal in the galvanic series, then the ions deposit on the cathode.

+ 2e- - cl (atcathode)

Factors influencing rate of corrosion.

A] Nature of metal

B] nature of environment

Nature of metal: -

Position of metal in galvanic series: -

• When 2 metals are in the contact in the presence of the electrolyte then more active metal undergo corrosion.

• Greater the difference in their positions, higher is the rate of corrosion.

Relative areas of anode and cathode: -

• If the anode < cathode then rate of corrosion.

• If the cathode < anode

Purity of metal: -

• Pure metal resists the corrosion

• Rate of corrosion increases with increases in of impurities which from galvanic cells.

Physical state: -

• Smaller the grain size of metal alloy greater the mate of corrosion.

e.g: - steel get corroded fast than the cost iron because

grain size of steel < grain size of cast ion.

Nature of oxide film: -

• If oxide layer – porous

Then corrosion take place

• If oxide layer – nonporous

    Then no corrosion takes place

Over – vtg (over – potential): -

• To restore position of metal some extra VTG is required is called as over –vtg

• Higher the over vtg less is the rate of corrosion.

e.g: -Zn,Pb,Cr,Ni, etc.

B] nature of environment: -

1) temperature: -

• Higher is temperature higher is the rate of dry corrosion because attacking gas and metal get activated @ higher temp.

• Higher is the temperature higher is rate of wet corrosion because: -

                 High tempo, according to nearest equation electrode potential is high.

2) moisture: -

• Higher is the moisture

          Higher is the rate of dry and wet corrosion

3) PH: -

• Generally acidic media have more corrosive effect on metals.

4) conductivity of medium: -

• High the electrical conductivity of aq. conducting medium higher is the rate of corrosion.

5) nature of ions: -

• Cl -, no3 – have the ability break nonporous oxide layer and cause wet corrosion.
• Presence of oxalate, m phosphate and silicate ions have the ability to slow down rate of wet corrosion.

Pour baix diagrams

Pour baix diagrams are the plots of: -

A] potentials of metal VS PH of media

B] rate of corrosion vs voltage applied

• Plot ai. e potentials v/j ph. it shows corrosion zone passivity zone and optimum PH at which metal has mi corrosion.
• Plot b rate of corrosion v/j vet it shows vtg range over which the metal is passive and resist the action of medium.

Merits: -

1. It is used to study corrosion phenomenon.
2. It also gives an into about how the corrosion is expected and can be minimized.
3. It is used to do study protection of metal from the corrosion.

Demerits: -

1. These diagrams are applicable to pure substances only.
2. These diagrams are derived for selected conc. of ionic species.

• Any two metals can be used to make a galvanic cell. Whether a metal will behave as an anode or a cathode in combination with another metal in the same soil environment, can usually be determined by its relative position on the galvanic series.
• The metal which appears higher up on the list (more negative) will, in general, be the anode and will thus corrode. The metal lower down on the list (more positive on the potential scale) will be the cathode and thus will not corrode. Of course, this galvanic action will not take place under open-circuit conditions; there must be a connecting circuit.
• Whenever a copper pipe service line is directly connected to a cast iron gas or water main, a galvanic cell such as shown in the following Figure is formed.
• The soil is the electrolyte, the copper service line is the cathode, the iron (or steel) main is the anode, and the connecting circuit is completed by attaching the line to the main.
• Normally, such cells do not do any great amount of damage, because the anode (corroding metal) is so much larger than the cathode that the attack is spread out over a large area.

One way to minimize the dissimilar metal interaction that causes the corrosion of the structural steel anchor is to break the electrical path between these two components by adding insulators.

• It is the deterioration of parts of a metal surface at different rates, due to the parts of the surface coming into contact with different concentrations of the same electrolyte. These differing concentrations result in some parts of the metal acquiring different electric potentials.
• Concentration cells occur when the concentration of electrolyte in contact with the metal is different in two contact locations. The extent of this corrosion reaction is proportionate to the difference in concentrations at contact points. It also varies with the type of electrolyte.
• If an area of the electrolyte close to the metal shows a lowered concentration of metal ions, the region has to turn anodic in comparison to different portions of metal surface. Thus, this part of the metal corrodes faster, so as to increase the local ion concentration in electrolyte.
• It is most prevalent in the presence of oxygen. When pure oxygen comes into contact with a wet metal surface, corrosion action is enabled. However, the corrosion is most severe in areas that have minimal oxygen contact.
• Parts of metal that are covered by scale will corrode faster because the contact with oxygen for these parts is restricted. Concentrated pitting can result due to this cumulative reaction.

Principle: -

             The metal to be protected is forced to behave as cathode.

2 types: -

A)    By using sacrificial anode

Methods / processes: -

• The metallic structure to be protected from corrosion is connected to the anodic metal by insulated wire.
• Anodic metal is more reactive like Zn etc.
• Anodic metal corrosion itself but it protects the cathodic metal from corrosion .it is called as sacrificial metal.
• Anodic metal is kept in back fill (mixture of coal and Nacl) to increase electrical contact with surrounding soil.
• When anodic metal consumed completely then it is replaced with fresh piece of metal

B)    By using impressed current: -

Method: -

• In this method an impressed current is applied in opposite to nullify the corrosion current and convert the corroding metal from anode to the cathode.

• The impressed current Is derived from DC source and given to insoluble anode like graphite, stainless steel which buried in soil.

• The –ve terminal of DC source is connected to pipeline to be protected from corrosion.

• Anodic metal is kept in black fill to increase electrical contact with the surrounding soil.

Limitations: -

• Highly investment and maintenance cost

• Special case needs to be taken to see the structure is not over protected otherwise H2 liberation maybe occur.

B] anodic protection: -

Principle: -

      A metal or alloy having wider range of passivity vtg, is made anode and the vtg is passivity range is applied over it do control its corrosion even by strong corroding media.

1. The metallic installation (reactor,water,tank, water coolers, industrial condensers,etc.) is made anode by using reference electrode and axillary electrode.
2. There is a potentiostatfor applying the desired ammeter to watch the corrosion current. the min or negligible current indicates that anodic protection take place successfully if the system goes out of corrosion control, then immediately remove all electrical connection.

Application: -

• Chemical reactors
• Industrial water coolers
• Complex metallic installations on ground or under sea water.
• Industrial metals condensers
• Pipelines for carrying corrosive liquids.

Advantages: -

• Low operating cost
• It is applicable for highly corrosive media
• Suitable to protect complex structures
• Protection current gives anidea about rate of corrosion.

Limitations: -

• High installation cost
• High starting current is req.
• In case system goes out of control then high is the rate of corrosion

It is applicable for only those metals which show active – passive behaviour.

Metallic coatings and its types

Metallic coating: -

Cleaning methods: -

2 types: -

1. Anodic coating
2. Catholic coating

Anodic coating: -

                         Coating metal is higher placed than the base metal then it is called as anodic coatings.

e.g.: -galvanizing

cathodic coatings: -

if coating metal is lower placed in galvanic series than base metal then it is called as tinning.

Methods of applying metallic coatings: -

• Hot dipping
• Electroplating
• Cementation’
• Metal cladding

Hot dippingprocesses: -

1)     Galvanizing: -

Method: -

• Coating of Zn on the steel or iron is called as galvanizing.
• Steel article is cleaned well with dil   H2SO4 then with water and dry it.
• Then the steel article is dipped in molten Zn bath and it is maintained at the temp 425 – 450 c.
• The molten Zn bath is covered with flux.
• Then the article is allowed to pass through to mate uniform coating layer and to remove excesses of Zn.

Applications: -

1. Various galvanized iron (G.I) articles are used.
2. It is useful for the protection of iron.

Tinning: -

Method: -

• Coating of tin on a base metal i.e steel iron is called as tinning.

• The steel article is cleaned well and then passes through molten tin bath which is maintained at temp 240c.

• Then, the article is allowed to pass through palm oil bath which protects the hot tin coated surface against corrosion.

• Then the article is allowed to pass through to make uniform coating layer to remove excess of tin.

Applications: -

1. Tin has greater corrosion resistivity and better glaze

2.     Food material can be stored in tin containers

3.     CV wire before involution by rubber are tinned.

4.     Tinned or brass vessels or sheets are used for making cooking vensiles and parts of reflectors.

• Electroplating: -

Principle: -

         Electroplating is a method in which coating metal is coated on the base metal on the basis ofelectrolysis principle.

Processes: -

• The article to be electroplated is cleaned well.

• There is a non-conducting tank which containing coating metal salts.

• The article is connected to the negative terminal of DC source which act as a cathode.

• Anode is a coating metal.

• After adjusting suitable PH and density on the base metal, then this method is started.

Metal ions in a solution migrate towards the article and get deposit on the base metal in the form of coating layer.

1. Cr + 3e-        -            Cr (if cr plating)
2. Ag+ + Ag         -          Ag (if Ag plating)
3. Ni + + 2e-           -        Ni (if Ni plating)

At anode (coatingmetal)

1. Cr -   Cr +     +     3e –
2.  Ag   -   Ag +   + e-
3. Ni       -   Ni +   + 2e-

Advantages: -

1. Electroplating can be done on the article of any shape
2. This is strong coating

Application: -

1. Corrosion protection method
2. Decoration
3. It is also applicable on the non – metallic surface
4. Electroplating is done on the many parts of machines.

• Coating metals: -Cu,Ni,Cr, Ag, etc.
• Base metals: - Fe (steel)non-metallic surface like glass.

• Cementation: -

             In this method the base metal articles are packed in the powder of coating metal and heated to temp. just below the M.P of lower melting component of them.

It is useful for making alloys of metals.

There are 3 types of cementation

1. Sherardizing – if coating metal is Zn or Fe
2. Colorizing – if coating metal is Al
3. Chromising – if coating metal is Cr

Application: -

• It is applicable for base metal like steel

• It is suitable for coating small articles like nuts,bolts, screws etc.

• This coating is strong with good abrasion resistivity

• Cladding:

Metal cladding: -

                     Metal cladding is a process in which a thin sheet of coating metal is bonded to the base metal on one or both the sides.

Clads: - coating metal sheets

Clad metals; -pb,Ni,Cu,Ag,Cr, al etc. and alloys like stainless steel,brass, nickel alloy, lead alloy.

Base metal: -brass, mild steel.

Method: -

• Base metal sheet is sandwiched b/w sheet of coating metals then this set of sheets is passed through the rollers under the action of heat and pressure.

• Clads fixed mechanically on the base metal.

Applications: -

• On regular surfaces of base metal
• For aeroplanes parts
• Doors and windows,etc.