STUDIES ON SOME ANTI-CORROSIVE PAINTS AND PAINT MEDIA

Abstract

In nsture elements generally occur as compoun is and hence the free elements are overrid lei by a tendency to revert to their natural stnte ( the most stable state bevlnf the least free energy ). This reve sion phenomenon is tenc«l as ' ^orroslon* which he»s been defined by ller1 'as destruction of a metal or en alloy by a cberslcal change, electrochemical change, or physical dissolution. ring corrosion, the free metal changes into on© of its stable compound, e.g., iron exposed to cist air gets cor verted into its brown hydrated oxide commonly known as rust. The phenomenon of corrosion hns been dealt at lw^th by Davy5*'1 end LaRive4. Ibwever, the modem theory of corrosion stems from the relatively recent work of hltney* *-nd Valker6*7. The major work done in the field has be«i reviewed by Miley and vans§ and «ller and Edwards** lo. A broad classification of corrosion processes Into * et' and * >ry* is now generally accepted and these terms are In common use. The term wet corrosion includes all reactions in which an aqueous solution Is involved in the reaction mechanism. The absence of water or an aqueous solution It implicit in dry corrosion. 2 Almost all metals tend to be coated vith a thin film of metallic oxide or oxygen, in some crses the adsorbed sulphide or other ions,within a minute of exposure to air. n films may render them ssore noble i.e., less susceptible to corrosion. Noble metals vhich occur In the virgin state in nature are ktown to be protected with a molecular layer of the oxide ''for example,gold end platinum). The mechanism of oxlde,ru^ Mde,bromlde or Iodide film formation has been shovn by ?}gner' N ,at ,a, }rune ! aid'* &n<* Mott,8» ,a» n to be el retro chemical in nature. *ince the 6*motion of the film involves I v-'Cmtof the LottO, (OrtfcOf hull-i I Of the Ua OOpeodl on the resistance of the film already formed. The actual process of oxidation cm be divided, rather arbitrarily, into three primary steps (i) the adsorption of oxygen from the gas phase, (ii) reaction and fonsation of a thin oxide Him, frequently referred to as tarnishing, and (111) the formstlon of a thick oxide layer or mill scale. The film of the oxide is often either discontinuous or unstable and if the metal is in contact with ©n electrolyte, electrically charged particles (positive ions ) tea4 to go i to the solution through the pores of the oxide film, "b ever, since the solution must remain olectrlcrlly neutral as a whole,these positive ions can enter the solution only if so equivalent number of positive ions of some other client are 61splenoma rbr example, a piece of iron Is ersed In copper sulphate solution undergoes this t./peof corrosion, copper being plated out. 'ater itself being an electrolyte, dissociates into the H4 and OH* ions. Metals,therefore, react with water to fonr the octal hydroxide and hydrogen, drogen is plated out as a thin invisible film on the metal surface. A typical, dectroches leal equation fbr this primary reaction c»n be written as hllewst M4 nH* * M* ♦ nH .... 1 Tn the case of Iron the general equation becomes, ?e ♦ 2R* >?e**+ 2H .... 2 Moisture in the atmosphere results as a consequence of rein,direct Impingement md condensation on the surfsee of various parti cles in air. In order to initiate a corrosion reaction, it is not eoser tial thrt the surface be exposed to an atmosphere henvily l&den with moisture. Hygroscopic dust,soll particles,corrosion products, coatings or rap rings adjacent to the surface may adsorb sufficient moisture to support the reaction. The liberated hydrogen interferes with the continuation of the reaction by (a) Insulating the base octal from the solution, and (b) r< turning the hydrogen in the film to the solution at an Increased rate due to 4 increase in its oo. centratlon at the surfoce. In m©ny cases the Influence may so reduce the rate of reaction that the corrosion products are not visible to the naked eye. Fbr the corrosion rose tl on to proceed, the film of hydrogen must be removed by some noons* Removal usually results by virtue of the hydrogen combining with oxygen (dissolved in the solution ) to fbrm water or by its escape as bubbles. 2H + ~ 0a j.HgO •••• 3 and 2R •NaT •••• 4 The removal of hydrogen causes more metal to go into solution, the rate of dissolution of the esctal being dependent on the rate of dissipation of drogen. In relatively neutral solutions the rate of dissipation of hydrogen Is determined by the concentration of the dissolved oxygen at the interface, Thus In the case of Iron, as a consequence of reactions (2) ,(3 end (4 ) , iron hydroxide is formed which on oxidation gives the red ferric oxide (ferric hydroxide on drying becomes the common rust, hydra ted ferric oxide ). In told solutions, the film Is usually removed as a gas. The tendency of hydrogen to plate out increases with the concentration of hydrogen Ions,resulting In many eooes, In t;><> romattoo or sotalea of ool solsr Oydrogoo. 5 In solutions containing other ions the principle is essentially the sane, although the actual mechanism depends on the interaction of various types of ions r resent. Thus in the presence of strong i lis, aluminium and other amphoteric tr.etals form alumina tea, etc. It has been shown that the reactions Involved during corrosion are electro chemical in nature. If one postulates a situation in the metal slnllof1 to that In a primary cell,there would be a flow of <r rrent in the circuit completed through the electrolyte end the metal would dissolve at the negative electrode (anode ). The rate of dissolution at any laatent will be deterla ed by the effective resistance of the circuit. This forms the foundations of the electrochrasrical theory pounded by .hitney5. The process taking place at the anode and the cathode can be rep resetted ast Metal M* oxide se, /node •I "iS Cathode Dry corrosion 9tBl Ma"*; oxygon —-• OTT 1 electrolyte solution mode • etal U tbodo Wet corrosion The adsorption of oxygon from the gas phase into the clean metal surface takes place with extreme 6 rapidity. Although chctni sorption is slow, ven then It plays a vital role In the oxidation of the metal, Pbr chemi sorp tlon of oxygen, 6-7 ev o f energy18 is required to dissociate the oxygen molecule m& it qpoemft that this energy can be available at certain specific sites In the metal, such as dislocations. The dissodction of one molecule at such a site disturbs the electron density in its neighbourhood. The probability of a second molecule being adsorbed and dissocisting adjacent to the first then becomes many times the probability of its adsorption at any arbitrary point on the plain surface of the metal. A nucleation of oxidation at the dislocations thus,sets In and spreads by a mechanism similar to crystal growth. This mechanism will,therefore, also influence the growth of oxide film. The processes taking place at the anode and the cathode in dry corrosion aret (1) at the anode interface* M• M^O/q ♦ 2(•"o/q ) where m Mw0 is the interstitial motel ion, e*0 is en Interstitial electron •ud IQ indicates the metal - oxide interface. 7 If the octal dissolve to enter a vacant rite theni M• M^o/q ♦ I 0*Q lQ where M^Q is a catton vacancy god o"q is a positive hole. (ii) at the cathode interface, i.e., at tho gas-oxide in terfacet (\- 0« /ado.) ♦ 2(e" /x ) e (0®m / ads.) , or the formation of Cb~ or 0" where /x is the gas- oxide interface. The corresponding 'Wet* corrosion half reactions aret (1) at the anode [«*♦ Z."]littl€# ♦ «*-«* ril) at the cathode Oa ♦ 2H«0 ♦ 4e" » 40H* or Oa ♦ 4ft* ♦ 4e* » 2RS0 Chemistry of t ro teetlon films Corrosion phenomenon can be explained with the help of the following reaction* 4Fe >4Pe"* ♦ 8o" Anodic reaction 2 Oa ♦ 4H*Q ♦ 8e~—»8 OH* Catho<Hc reaction or 4Fe ♦ 2 08 ♦ 4HaO—>4Fe(0H ) a—» I Qifi§ xU^) (But) Rust formation la accompanied by a flow of electrons 8 from anodic to eathodlc regions and movement of ions in solution. Thus, In order to inhibit corrosion, it is necessary to stop the flow of current. This can bo achieved by suppressing either the cstbodic or anodic reaction, or by inserting a high resistance in the electrolytic path of too corrosion current. The three methods of aupj resflon can be* (1) ca tho die inhibition (ii) anodic inhibition (ill) resistance inhibition Point fllirs are the most common end convenient an ti-corrosive agents. Feint films have a marked influence on the corrosion reactions and their structure goes s long vay to determine the extent of an tl -corrosi vo action. ©Lock and Harris1* from their studies on the adsorption of krypton on dried unsupported paint films based on linseed oil have concluded that paint fiima are free from pores. I hey have calculated the surface area of the films and found that tho films had a low roughness factor of 1.14 as against 1.30 for smooth, polished stool surface. They have further shown toot pores developed in the film when the pigment loading on the paint approached the critical •ment volume concentretlon (PVC). Sktcnslvo examination of dry paint films under an electron microscope have been carried out by Fobaleka°»a *, Bell** and Taith8*. Their 9 studies also show that paint films are free from pores or capillaries when dry and develop these when the film expands owing to water uptake. Infotmation regarding the structure can also bo obtained from the rates of diffusion of gases through polymer films. If diffusion tnkes place through capillaries, the Graham* a law should hold good. Northrop84 measured the permeability of a number of gases through dry cellulose nitrate membranes and fbund that the values were not in the inverse order of the square roots of their densities. hen the.membranes were saturated with water the diffusion values were not ap r redably affected. Morthrop considered this as s strong evidence for the absence of capillaries, particularly in dry films, because the rates of diffusion of gases through gases is over 10,000 times greater than the rotes of diffusion of gases through liraids. Ho concluded that the gases dissolve in the membrane and move under a concentration gradient and evaporate on the other side. . Further information een also be obtained from the behaviour of paint films when immersed in water and in solutions of electrolytes. As early as 1923, Morrell89 had jointed out that the presence of electrolytes inhibit the absorption of water by vamishea. Stronger the salt solution. 10 lesser is the water ur take by the varnish film. The behaviour of a two cont "Ikyd system when immersed in solutions of sucrose and sodium chloride was studied by tteiberfer and flm**. They found the weight increase of the film to be independent of the nature of the solute but dependent on the osmotic pressure of the s lution. They further found that after 35 days, the films immersed in water were still gaining weight but the weights of tho films Immersed in various solutions had become almost constant. The increase in weight could not so ascribed as due to filling of carillsries, because if such were the ease, then It should be independent of the presence of the solute. These results indicate that the swelling controlled by osmosis plays an important part in the absorrtlon of water by feint films. It has been reported that when a current passes through a film of polystyrene*", mdosmotic transfer of water ions take place. This observation has been extended by virth and Machu8* to a range of paints based on linseed oil. To account Ibr the endosmosls phenosnenon, two assumptions h;ve to be madef firstly, the film contains a net work of fixed charges and secondly, the film contains a capillary structure. Capillary structure gets filled up with water when on ion moves through the ElIn as it drags 11 along an appreciable number of water molecules. They have further concluded that the membranes are having pores of radius between 1 xlO*8 m<\ 3 xlO** mm. rt is,however, still by no means clear whether the water contained in nt films exists in the form of continuous pr*ths through the film or It is dispersed in an in tensed! ate mann er.