effect on metal coupling on rusting of iron

8/9/2019 effect on metal coupling on rusting of iron

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*hen impure !cast" iron is incontact with water, oxygen, or other strong oxidants, or acids, it rusts.

If salt is present, for example in seawater or salt spray, the iron tends to rust more quickly,as a resultof electrochemical reactions. Iron metal is relatively unaffected by pure water or by dryoxygen. +s withother metals, like aluminium, a tightly adhering oxide coating, a passivation layer,

protects the bulk ironfrom further oxidation. he conversion of the passivating ferrous oxide layer to rustresults from thecombined action of two agents, usually oxygen and water.

Other degrading solutions are sulfur dioxide in water and carbon dioxide in water. -nderthese corrosiveconditions, iron hydroxide species are formed. -nlike ferrous oxides, the hydroxides donot adhere to the

bulk metal. +s they form and flake off from the surface, fresh iron is exposed, and thecorrosion processcontinues until either all of the iron is consumed or all of the oxygen, water, carbon

dioxide, or sulfurdioxide in the system are removed or consumed. he rusting of iron is anelectrochemical process that begins with the transfer of electrons from iron tooxygen. he iron is the reducing agent !gives up electrons" while the oxygen is the ox i dising agent !gainselectrons". he rate of corrosion is affected by water and accelerated by electrolytes, as

illustrated by theeffects of road salt on the corrosion of automobiles. he key reaction is the reduction ofoxygenO$ / 0e1 / $'$O 2 0O'13ecause it forms hydroxide ions, this process is strongly affected by the presence of acid.Indeed, thecorrosion of most metals by oxygen is accelerated at low p'. 4roviding the electrons for


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3ecause of the widespread use and

importance of iron and steel products, the prevention or slowing ofrust is the basis of ma5or economic activities in a number of speciali6ed technologies. +

brief overview ofmethods is presented here7 for detailed coverage, see the cross(referenced articles.Rust is permeable to air and water, therefore the interior metallic iron beneath a rust layer continues tocorrode. Rust prevention thus requires coatings that preclude rust formation.


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tainless steel forms a passivation layerof chromium!III" oxide. imilar passivation behavior also readilyoccurs with magnesium, titanium, 6inc, 6inc oxides, poly aniline, and other electro active

conductive polymers.pecial 8weathering steel8 alloys such as 9or( en rust at a much slower rate than normal,

because therust adheres to the surface of the metal in a protective layer. :esigns using this materialmust includemeasures that avoid worst(case exposures, since the material still continues to rust slowlyeven undernear(ideal conditions.


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@ 9orrosion is a serious problem of some metals like iron, 6inc, al u minium and alloyslike brass which arecommonly used in day to day life.

@ +part from reducing the life of articles made up of these metals or alloys the chemicalsubstances formed out ofcorrosion have serious public health problems.

@ Replacement of machines or their parts and many other articles in industrial and publicdealing lead to hugeexpenditure.

@ 'ence, how to reduce or avoid corrosion of articles made up of metals or alloys has been a ma5or sub5ect of

study in the field of chemistry and electro(chemistry.


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!i" )ix A ml.of conc. ' 9lwith $0; ml.of water to

form $


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;. he rate ofcorrosion observed in acidic medium or the mass consumed during the corrosion is in thedecreasing order from brass to aluminum. 3rass has the highest corrosion rate whilealuminium has the least corrosion rate. Brass > Iron > Zinc > Aluminium

$. *hen coupling of these metals was done eachcouple showed some difference in their corrosion with respect to each metal kept alone.Iron / +luminium couple has the highest rate of corrosion while iron />inc couple hasthe lowest rate of corrosion. Rate of corrosion of each couple is in the order of Iron +Aluminium > Brass + Zinc> Iron + Zinc

%. Rate of corrosion in basic medium is in thedecreasing order from 3rass to +luminium. he order of rate of corrosion is as belowBrass > Zinc >Iron > Aluminium

0. *hen these metals were coupled the rate of corrosionwas in the decreasing order from Brass+ Aluminium > Brass + Zinc > Iron +Aluminium


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@ 9orrosion is a serious problem of some metals like iron, 6inc, a l u minium and alloys like brass which are commonly used in day to day life. +part from reducingthe life of articles made up of these metals or alloys the chemical substances formed outof corrosion have serious public health problems. Replacement of machines or their partsand many other articles in industrial and public dealing lead to huge expenditure.@ 'ence, how to reduce or avoid corrosion of articles made up of metals or alloys has

been a ma5or sub5ect of study in the field of chemistry and electro(chemistry.

@ he study of the rate of corrosion of different metals or alloys showed gradual decreasein their masses in acidic medium. he decrease is in the order of brass


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Rust is associated with degradation of iron(based tools and structures. +s rust has a muchhigher volume than the originating mass of iron, its build(up can also cause failure byforcing apart ad5acent parts F a phenomenon sometimes known as 8rust packing8. It wasthe cause of the collapse of the )i anus river bridge in ;AD%, when the bearings rustedinternally and pushed one corner of the road slab off its support. Rust was also animportant factor in the ilver 3ridge disaster of ;A=C in *est Girginia, when a steelsuspension bridge collapsed in less than a minute, killing 0= drivers and passengers onthe bridge at the time.he Hi n 6 u a 3ridge in 4ennsylvania was blown down by a tornado in $BB%, largely

because the central base bolts holding the structure to the ground had rusted away,leaving the bridge anchored by gravity alone.Reinforced concrete is also vulnerable to rust damage. Internal pressure caused byexpanding corrosion of concrete(covered steel and iron can cause the concrete to spall,creating severe structural problems. It is one of the most common failure modes ofreinforced concrete bridges and buildings.


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Rust formation can be controlled with coatings, such as paint, lacquer, or varnish thatisolate the iron from the environment. arge structures with enclosed box sections, suchas ships and modern automobiles, often have a wax(based product !technically a8slushing oil8" in5ected into these sections. uch treatments usually also contain rustinhibitors. 9overing steel with concrete can provide some protection to steel because ofthe alkaline p' environment at the steel(concrete interface. 'owever rusting of steel inconcrete can still be a problem, as expanding rust can fracture or slowly 8explode8concrete from within.+s a closely related example, iron bars were used to reinforce stonework of the 4arthenonin +thens, Jreece, but caused extensive damage by rusting, swelling, and shattering themarble components of the building.*hen only temporary protection is needed for storage or transport, a thin layer of oil,grease, or a special mixture such as 9osmoline can be applied to an iron surface. uchtreatments are extensively used when 8mothballing8 a steel ship, automobile, or otherequipment for long(term storage.pecial anti(sei6e lubricant mixtures are available, and are applied to metallic threads andother precision machined surfaces to protect them from rust. hese compounds usuallycontain grease mixed with copper, 6inc, or aluminum powder, and other proprietaryingredients.


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9athodic protection is a technique used to inhibit corrosion on buried or immersedstructures by supplying an electrical charge that suppresses the electro(chemical reaction.If correctly applied, corrosion can be stopped completely. In its simplest form, it isachieved by attaching a sacrificial anode, thereby making the iron or steel the cathode inthe cell formed. he sacrificial anode must be made from something with a more negativeelectrode potential than the iron or steel, commonly 6inc, a l u minium, or magnesium.he sacrificial anode will eventually corrode away, ceasing its protective action unless itis replaced in a timely manner. 9athodic protection can also be provided by using aspecial(purpose electrical device to appropriately induce an electric charge.


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effect on metal coupling on rusting of iron