Seminar

Corrosion!Corrosion!

Corrosion is the gradual destruction of material, Corrosion is the gradual destruction of material, usually metal>>this mean oxidation of metals with usually metal>>this mean oxidation of metals with oxygen.oxygen.

Corrosion occurs in the presence of moisture. For Corrosion occurs in the presence of moisture. For example when iron is exposed to moist air, it example when iron is exposed to moist air, it reacts with oxygen to form rust reacts with oxygen to form rust (Fe2O3.XH2O).(Fe2O3.XH2O).

i.e., rust is hydrated iron (III) oxidei.e., rust is hydrated iron (III) oxide

Corrosion is a natural Corrosion is a natural process…..tends to the lowest process…..tends to the lowest possible energy states.possible energy states.

e.g: Iron has a natural tendency to e.g: Iron has a natural tendency to combine with other chemical combine with other chemical elements( such as oxygen) to elements( such as oxygen) to return to lower E states:return to lower E states:

WET & DRY CORROSIONWET & DRY CORROSION

Wet / aqueous corrosionWet / aqueous corrosion is the major form of is the major form of corrosion which occurs at or near room corrosion which occurs at or near room temperature and in the presence of watertemperature and in the presence of water

Dry / gaseous corrosionDry / gaseous corrosion is significant mainly is significant mainly at high temperaturesat high temperatures

The hot dip galvanizing processThe hot dip galvanizing process


Loading-jiggingLoading-jigging

The steel items need to be supported during the hot dip The steel items need to be supported during the hot dip galvanizing process.galvanizing process.

Can the item be easily jigged?Can the item be easily jigged?Will touch marks be a problem?Will touch marks be a problem?

Solutions and molten zinc must be able to flow into all Solutions and molten zinc must be able to flow into all sections and drain off all sections.sections and drain off all sections.

Steel items must be designed to prevent air,solutions and Steel items must be designed to prevent air,solutions and molten zinc becoming trappedmolten zinc becoming trapped


Degreasing Degreasing A hot (90 C) caustic bath is used to remove oil,grease,paint A hot (90 C) caustic bath is used to remove oil,grease,paint another organic compoundsanother organic compounds

If these contaminating materials are not removed,the next If these contaminating materials are not removed,the next stage of the process(pickling) is affectedstage of the process(pickling) is affected

Check if any paint is on item-some paints are not easily Check if any paint is on item-some paints are not easily removed and may require abrasive blasting before galvanizingremoved and may require abrasive blasting before galvanizing

some pipes and pipe fittings have a black varnish coating that some pipes and pipe fittings have a black varnish coating that cannot be removed in the caustic bath.cannot be removed in the caustic bath.


Acid pickling Acid pickling

the steel items are immersed in hydrochloric acid to remove-the steel items are immersed in hydrochloric acid to remove-rust , mill scale and other metal oxidesrust , mill scale and other metal oxides

The steel surface must be perfectly clean of these oxides for The steel surface must be perfectly clean of these oxides for the molten zinc to react with the steelthe molten zinc to react with the steel

very heavy rust may not be easily removed by pickling badly very heavy rust may not be easily removed by pickling badly rusted items should be abrasive blasted or mechanically rusted items should be abrasive blasted or mechanically cleaned first.cleaned first.


Pre-fluxingPre-fluxing

steel items are water rinsed after pickling and steel items are water rinsed after pickling and immersed in hot (70-80 C)zinc ammonium immersed in hot (70-80 C)zinc ammonium chloride(ZAC)solution.chloride(ZAC)solution.

The ZAC solution conditions the clean steel surface The ZAC solution conditions the clean steel surface ready for hot dip galvanizingready for hot dip galvanizing

Good pre-treatment = good quality galvanizingGood pre-treatment = good quality galvanizing

The hot dip galvanizing processThe hot dip galvanizing process Hot dip galvanizingHot dip galvanizing

The steel is immersed in molten zinc(temp 450 C)The steel is immersed in molten zinc(temp 450 C)The clean steel surface reacts with the molten zinc to form a zinc-iron alloy The clean steel surface reacts with the molten zinc to form a zinc-iron alloy which is very strongly bonded to the surface.which is very strongly bonded to the surface.

The hot dip galvanized coating forms in 3-5 minutes , depending on the The hot dip galvanized coating forms in 3-5 minutes , depending on the steel thickness.steel thickness.


QuenchingQuenching

After hot dip galvanized steel item is removed from After hot dip galvanized steel item is removed from the galvanizing bath , it is immediately quenched in a the galvanizing bath , it is immediately quenched in a sodium dichromate solutionsodium dichromate solution

The dichromate quenching cools the item so that it The dichromate quenching cools the item so that it can be quickly handled and conditions the surface of can be quickly handled and conditions the surface of the galvanized coating to maintain its bright the galvanized coating to maintain its bright appearance.appearance.


CharacteristicsCharacteristics

Typically 65-150 microns thick (1mm=1000microns)Typically 65-150 microns thick (1mm=1000microns)Zinc iron alloy layers make up 80% -100% of the coating.Zinc iron alloy layers make up 80% -100% of the coating.The zinc-iron alloy layers are harder than 250 grade steel The zinc-iron alloy layers are harder than 250 grade steel

Micrograph of hot-dip galvanized coatingMicrograph of hot-dip galvanized coating


Coating comparisonsCoating comparisons

Compared to zinc electroplating(ZEP) and continuous Compared to zinc electroplating(ZEP) and continuous galvanizing (CG),hot dip galvanizing (HDG) is better galvanizing (CG),hot dip galvanizing (HDG) is better because….because….-HDG is thicker and will last over 50 years or more without -HDG is thicker and will last over 50 years or more without rustingrusting-HDG is harder and is 5 times more abrasion resistant than -HDG is harder and is 5 times more abrasion resistant than ZEP and CGZEP and CG-HDG coats all external and internal surfaces and edges . All -HDG coats all external and internal surfaces and edges . All CG coatings have cut , exposed edges after processing.CG coatings have cut , exposed edges after processing.

Coating thickness of coating on Coating thickness of coating on galvanized productsgalvanized products

Corrosion removalCorrosion removal

It is possible chemically to remove the It is possible chemically to remove the products of corrosion to give a clean products of corrosion to give a clean surface. For example surface. For example phosphoric acid. . Phosphoric acid may be used as a Phosphoric acid may be used as a "rust converter", by direct application "rust converter", by direct application to rusted iron, steel tools, or to rusted iron, steel tools, or surfaces. The phosphoric acid surfaces. The phosphoric acid converts reddish-brown iron(III) converts reddish-brown iron(III) oxide, Fe2O3 (rust) to black ferric oxide, Fe2O3 (rust) to black ferric phosphate, FePO4.phosphate, FePO4.

Corrosion of metal can be prevented if the metal is Corrosion of metal can be prevented if the metal is coated with something which does not allow moisture coated with something which does not allow moisture and oxygen to react with itand oxygen to react with it..

Coating of metals with paint, oil, grease or Coating of metals with paint, oil, grease or varnish prevents the corrosion of metals.varnish prevents the corrosion of metals.

Coating of corrosive metals with non-corrosive Coating of corrosive metals with non-corrosive metals also prevents corrosion. Some of the metals also prevents corrosion. Some of the methods by which metals can be coated with non-methods by which metals can be coated with non-corrosive metals are:corrosive metals are:

Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode.

ElectroplatingElectroplating

The process used in electroplating is called electrodeposition. It is analogous to a galvanic cell acting in reverse. The part to be plated is the cathode of the circuit.

ElectroplatingElectroplating

ElectroplatingElectroplating: In : In this method of a metal is this method of a metal is covered with another covered with another metal using electrolysis. metal using electrolysis. Silver-plated spoons, Silver-plated spoons, gold-plated jewelry, etc, gold-plated jewelry, etc, are electroplated.are electroplated.

Silver electroplating Silver electroplating

cellcell>>> >>> The anode is The anode is a silver bar and the a silver bar and the cathode is an iron cathode is an iron spoon>>>spoon>>>

Polymer,heal thyselfPolymer,heal thyself

A new polymer-based A new polymer-based system that can heal it self system that can heal it self when it becomes when it becomes damaged….the material damaged….the material relies on network of vessels-relies on network of vessels-similar to blood similar to blood capillaries>>that carry a capillaries>>that carry a healing agent to areas that healing agent to areas that damaged.damaged.

Polymer,heal thyselfPolymer,heal thyself

The healing agent –monomeric The healing agent –monomeric dicyclopentadiene….fill into 200 dicyclopentadiene….fill into 200 micrometer channels. micrometer channels.

the catalys:the catalys:

Benzyldienebis(tricyclohexylphosphine)dichloBenzyldienebis(tricyclohexylphosphine)dichlororutheniumincorporated into the outer rorutheniumincorporated into the outer coating.coating.

When a damage occurs at the coating, When a damage occurs at the coating, healing agent wicks from the healing agent wicks from the

microchannels into the crack(s) through microchannels into the crack(s) through capillary action>>>then the healing agent capillary action>>>then the healing agent interacts with the catalyst particles in the interacts with the catalyst particles in the

coating to initiate polymerization , coating to initiate polymerization , rebonding the crack faces autonomicallyrebonding the crack faces autonomically

The reaction between monomer and catalyst The reaction between monomer and catalyst to produce solid compoundto produce solid compound

After a sufficient time period the cracks are healed and the structural integrity of the coating restored.

The catalyst lowers the energy barrier of the reaction and allows the monomer to polymerize without the addition of heat.

There are many challenges in designing this type of material:

1) the reactivity of the catalyst must be maintained even after it is enclosed in wax

2) the monomer must flow at a sufficient rate (have low enough viscosity) to cover the entire crack before it is polymerized

3) the catalyst must quickly dissolve into monomer in order to react efficiently and prevent the crack from spreading further.

Some areas affected by corrosionSome areas affected by corrosion

General Corrosion ExamplesGeneral Corrosion Examples

Collapse of Berlin Congress Hall Collapse of Berlin Congress Hall in 1980in 1980

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