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Selection of an appropriate corrosion inhibitor, inhibitor combination or package is an exceptionally cost ...

Selection of an appropriate corrosion inhibitor, inhibitor combination or package is an exceptionally cost
effective and materials saving measure in various industries.

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  • 1. Recent Patents on Corrosion Science, 2011, 1, 63-71 632210-6839/11 $100.00+.00 2011 Bentham Science PublishersContemporary Developments in Corrosion Inhibitors - Review of PatentsViswanathan S. Saji*School of Energy Engineering, Ulsan National Institute of Science and Technology, Ulsan 689-798, Republic of KoreaReceived: 25 March 2011; Revised: 04 April 2011; Accepted: 06 April 2011Abstract: Selection of an appropriate corrosion inhibitor, inhibitor combination or package is an exceptionally costeffective and materials saving measure in various industries. The article reviews recent (2008-2010) patents reported oncorrosion inhibitors. Up-to-date developments in the inhibition technology are presented in accordance with the areas ofapplication of inhibitors - industrial cooling water systems, oil and gas fields, coatings, acid pickling, lubricants, vaporphase inhibition, concrete corrosion etc. Nanotechnological advancements in inhibitor technology (in coatings) and novelproducts employing vapor phase corrosion inhibitors were discussed.Keywords: Corrosion inhibitors, cooling water systems, oil and gas field, paint and coatings, smart coatings, vapor phaseinhibition.INTRODUCTIONIn spite of much advancement in the field of corrosionscience and technology, the phenomenon of corrosion(mainly of Fe, Al, Cu, Zn, Mg and their alloys) remains amajor concern to industries around the world. Though theserious consequences of corrosion can be controlled to agreat extent by selection of highly corrosion resistantmaterials, the cost factor associated with the same, favors theuse of cheap metallic materials along with efficient corrosionprevention methods in many industrial applications. In thisaspect, corrosion inhibitors have ample significance asindividual inhibitors or as a component in chemicalformulations. They have wide commercial applications suchas in cooling waters, oil and gas fields, paint pigments,lubricants etc. [1].A large number of corrosion inhibitors have beendeveloped and used for application to various systemsdepending on the medium treated, the type of surface that issusceptible to corrosion, the type of corrosion encountered,and the conditions to which the medium is exposed [2]. Theefficiency andusefulness of a corrosion inhibitor under oneset of circumstances often does not imply the same foranother set of circumstances. In many applications likeindustrial water treatment systems, a combination of morethan one corrosion inhibitor along with other additives suchas anti-scalents, biocides and polymeric dispersants is supp-lied. In many areas, corrosion inhibitors work synchronouslywith superior coatings and cathodic protection. In spite of thelong history of corrosion inhibition; a highly efficient anddurable inhibitor that can completely protect iron and lowcarbon steel in aggressive environments such as highchloride electrolyte for longer duration is yet to be realized.The present article reviews recent (2008-2010) patentsreported on corrosion inhibitors. The manuscript brings light*Address correspondence to this author at the School of EnergyEngineering, Ulsan National Institute of Science and Technology, Ulsan689-798, Republic of Korea; Tel: +82-52-217-2963; Fax: +82-52-217-2929;E-mail: vssaji@unist.ac.krto the major outputs in the field of inhibitor technology inthe last two years and are presented according to theapplication area of inhibitors.CORROSION INHIBITION IN COOLING WATERSYSTEMSCorrosion in an open cooling water system is closelyrelated to mineral scale formation, solid deposition andmicrobiological fouling. Hence a comprehensive coolingwater additive composition consists of anti-scaling agents,polymeric dispersants and biocides along with corrosioninhibitors to protect Fe, Cu, Al and their alloys [3-5]. Highcontent of salt and dissolved oxygen increases the severity ofcorrosion in open systems. Galvanic corrosion is anotherserious concern. A successful water treatment is highlyeconomic by reducing inspection shutdowns and permittingextensive use of low carbon steel.The conventional inorganic inhibitors continue to be abest choice in inhibitor packages. Many of the recent patentscontain inhibitors such as silicates, phosphates, molybdatesand tungstates as a component in the proposed combination.Azole based compounds continue to be in the frontline as Cucorrosion inhibitor. Zhang et al. patented an inhibitorpackage of sodium silicate, sodium tungstate, sodiumphosphate and benzotriazole/mercaptobenzothiazole alongwith an acrylic co-polymer for desalted circulating coolingwater system [6]. Xiao et al. patented a scale and corrosioninhibitor composition for circulating cooling water systemthat comprised of 2-phosphonobutane-1,2,4-tricarboxylicacid, hydroxyethylidene diphosphonic acid, zinc sulphate,benzotriazole, sodium hexametaphophate and a co-polymer[7]. However the inorganic inhibitors such as phosphate andchromate are steadfastly becoming the object of federal andlocal regulations due to environmental concerns. Gernon etal. proposed a method of controlling corrosion of Fe and Cuusing alkylalkanolamine based compounds. The compoundhas the general formula Rx-NHy-(CH2CH2OH)z, where R isC3 to C8 alkyl groups; x and z are 1 or 2; y is 0 or 1 [8].Iyasu et al. patented thioglycolic acid salts and azolecompounds as pitting inhibitors for Cu substrates [9]. An
  • 2. 64 Recent Patents on Corrosion Science, 2011, Volume 1, No. 1 Viswanathan S. Sajiinhibitor combination of sodium gluconate, benzotriazole,tolyltriazole, molybdate, tungstate and zinc sulphate waspatented by Cao [10]. Liu et al. patented an inhibitorcombination of sodium tungstate (40 mg/L), poly(epoxy-succinic acid) (40 mg/L), sodium gluconate (10 mg/L) andzinc sulphate (4 mg/L) for circulating cooling seawater [11].Increased popularity of high alkalinity, no pH controlwater treatment programs have resulted in more frequent andsevere white rust corrosion issues, especially in coolingtower applications. Kidambi et al. patented a method forinhibiting white rust corrosion on galvanized surfaces byintroducing an effective amount of a corrosion inhibitingcomposition that includes one or more thiols basedcompounds [12]. Corrosive attack of Cu by NH3 in water is awell known scenario in the water treatment industry, whichpose problems in the reuse of wastewater sources containingNH3. Duke and Kubis patented a method of inhibition in thisdirection that was directed toward the use of azole basedinhibitors. Dosages of the azole or a mixture of two or moreazoles is proportional to NH3/NH4+ion concentration in thecooling water and to increasing TDS concentration andhigher pH level, preferably from 0.5 to 60 mg/L residual[13]. A corrosion inhibitor solution containing poly(epoxy-succinic acid) (2 mg/L), zinc sulfate (20 mg/L), and sodiumsilicate (10 mg/L) has been reported to have 93.05%inhibiting efficiency for white copper [14].For closed cooling water systems (e.g. engine coolants),the problems associated with scaling are very less. Heattransfer fluids, which generally comprise water, glycol orglycol-water mixtures, are in communication with one orseveral metallic parts that are prone to corrosion. Al isespecially vulnerable to corrosion and many vehicles haveheads, radiators and other Al components in the coolingsystem. Examples of optional conventional corrosioninhibitors include alkali metal borates, alkali metal silicates,alkali metal benzoates, alkali metal nitrates, alkali metalnitrites, alkali metal molybdates, and hydrocarbyl thiazoles.In a recent patent, Kawaguchi disclosed tourmaline basedcorrosion inhibitors for antifreezing fluids [15]. A corrosioninhibiting composition of monocarboxylic acid (octanoicacid), dicarboxylic acid (sebacic acid), triazole andtriethanolamine compound was patented by Belokurova [16].An antifreeze composition of triethanolamine phosphate, anon ferrous metal inhibitor, disodium salt ofethylenediaminetetraacetic acid and an antifoaming agentwith ethylene glycol balance was disclosed by Belikov [17].Zou patented an inhibitor composition for circulating andcooling system of automobiles that contains sodium nitrite,borax, 1,2,3-benzotriazole and zinc nitrate [18]. Fukutanipatented an aqueous solution of glycerin, saturated fattyacids/salts, saturated di/tricarboxylic acids/salts, tolyltriazoleand benzotraizole for use in radiator coolants [19]. Yuan etal. patented an additive composition of a monobasic organicacid, phytic acid, borax, hydroxyphenylhydrazine,carbohydrazide, polyacrylamide, siloxane, benzotriazole, anda siloxane stabilizer for circulating cooling water system ofinternal combustion engines [20]. Xie et al. patented aninhibitor composition based on bis-3 silane compounds (e.g.bis[3-(triethoxy-silyl)propyl]amine) that can be used forcorrosion of Mg alloys in engine coolants [21].Brazed heat exchangers are lower in weight and are ableto radiate heat better than heat exchangers formed bymechanical expansion. Yang et al. patented a heat transferfluids intended for use in heat transfer systems comprisingbrazed metals or metal alloys such as brazed Al. Thecorrosion inhibitor comprises a combination of inhibitorsselected from the group consisting of an azole-basedinhibitor, a siloxane-based inhibitor, a silicate, a carboxylate,a tall oil fatty acid, a borate, a nitrate, a nitrite, and an aminesalt. In general, the corrosion inhibitors are present in anamount of about 0.01 to about 10 wt%, based on the totalweight of the heat transfer fluid [22].CORROSION INHIBITION IN OIL AND GAS FIELDThe inhibition in oil and gas field is more complicatedand requires specialty inhibitors depending on the area ofapplication such as in refineries, wells, recovery units,pipelines etc. Aggressive gases such as H2S, CO2, andorganic acids complicate the problem of inhibition in wells.Corrosion problems in petroleum refining operationsassociated with naphthenic acid constituents and sulfurcompounds in crude oils have been recognized for manyyears. It is particularly severe in atmospheric and vacuumdistillation units at higher temperatures, 175-400oC.Dissolved O2 is the main species causing corrosion inrecovery units. Dry corrosion is of great importance inrefinery processes. HCl may form in refineries as a by-product. O2, CO2 and H2S intensify corrosion problems innatural gas pipelines.Wet corrosion in refineries can be controlled bypassivating, neutralizing or adsorption type inhibitors. Slaginhibitors are used along with the corrosion inhibitors toreduce deposits. Both water soluble and oil soluble inhibitorsare used in pipelines. Adsorption type inhibitors are widelyused for preventing internal corrosion of pipelines carryingrefined petroleum products. In general, where the operatingtemperatures and/or the acid concentrations are higher, aproportionately higher amount of the corrosion inhibitorcomposition will be required. It is preferable to add theinhibitor composition at a relatively high initial dosage rate,about 2000 to 5000 ppm, and to maintain this level for arelatively short period of time until the presence of theinhibitor induces build-up of a corrosion protective coatingon the metal surfaces. Once the protective coating isestablished, the dosage may be reduced to an operationalrange, about 10 to 100 ppm. It is known that nitrogen basedcorrosion inhibitors are relatively ineffective in the hightemperature environment. Also, the phosphorus-containingcompounds may impair the function of various catalysts usedto treat crude oil.Catalytic polymerization of tall oil fatty acids such asoleic and linoleic acids give varying amounts of dimerizedand trimerized fatty acids. These dimer and/or trimer fattyacids may be neutralized with an appropriate amine; whichthe oil industry has traditionally employed as an oil-solubleinhibitor for reducing corrosion in oil well piping and relatedrecovery equipment. Over the years, the corrosion inhibitionart has looked for alternatives to the dimer/trimer acid-basedproduct. Of particular interest in this regard is the class offatty acid-based products which have been functionalized
  • 3. Contemporary Developments in Corrosion Inhibitors Recent Patents on Corrosion Science, 2011, Volume 1. No. 1 65with maleic anhydride and/or fumaric acid. The fatty acidscan first be maleated followed by an oxidation.Alternatively, the fatty acid material can first be oxidizedand then the oxidized fatty acid product can be maleated[23]. Zagidullin et al. patented a terephthalic acid basedmethod for corrosion inhibition. An acid inhibitor isprepared by interaction of polyethylene-polyamine withterephthalic acid at 150-190 oC for 4-8 h in the molar ratio of2:1, followed by reaction with benzyl chloride at 80 oC for 5h. The product was used as a component along withurotropin and neonol in water [24]. Rhodanine (2-thioxo-4-thiazolidinone) and its 3- and 5- derivatives are patented as aFe corrosion inhibitor for oil refining equipment in carbonicacid derivatives [25].Zetlmeisl et al. disclosed a method for inhibitingnaphthenic acid corrosion at high temperatures based onnovel thiophosphorus compounds [26]. Subramaniyan et al.patented a polyisobutylene phosphorous sulfur compound ascorrosion inhibitors for naphthenic acid and sulphurcorrosion. This results in improved performance as well as adecreased phosphorus requirement. The compound isobtained by reaction of high reactive polyisobutylene withP2S5 in the presence of sulphur powder [27]. The authorrecently patented a novel polymeric additive (polymericphosphate ester of polyisobutylene succinate ester) as hightemperature naphthenic acid corrosion inhibitor [28]. Huanget al. patented a reaction product of thiophosphate (producedby reaction of phosphide with fatty alcohol) with spiro-diester (product of reaction of alkenyl succinic anhydridewith organic amine using inorganic salts as catalyst) as hightemperature corrosion inhibitor [29]. Alykov et al. patented1-nitro-3,3-diphenyl-1-[3-(3-nitrophenyl)-1,2,4-oxydiazol-5-yl]-2,3-diazoprop-1-ene, that was obtained by condensationreaction of equimolar quantities of substituted 3-aryl-5-nitromethyl-1,2,4-oxadiazol with 1,1-diphenylhydrazine inethoxyethane medium; as corrosion inhibitor for metalprotection against acid corrosion in oil and gas pipelines fornon-alloyed steels [30]. Strak et al. patented a method forinhibiting corrosion in a separation unit that comprisingtreating the unit with inhibitors selected from the groupconsisting of dodecenyl succinic acid, and di-hexyl succinicacid [31]. Leinweber patented low toxic, water soluble andbiodegradable corrosion inhibitors of metal salts ofCH3SCH2CH2CH(NHCOR)COOH containing anionic andcationic surfactants [32, 33].Bhat et al. disclosed a non chromate aqueous corrosioninhibitor composition for metallic surfaces exposed tomedium of pH 4, comprising of an anodic corrosion inhibitor(ammonium heptamolybdate/sodium orthovanadate), acathodic corrosion inhibitor (cerium chloride) and a metalcomplexing ligand (trisodium citrate-2-hydrate); dissolvingthe blended product in water as 2.5 wt.% solution [34]. N-acetyl-2-(2,3-dihydroxycilopentenyl) aniline with concen-trations 50-200 mg/L was proposed as an inhibitor of H2Scorrosion in mineralized water petroleum solutions [35]. 2-propyl-3-ethyl-8-oxychinoline-ZnCl2 complex was disclosedas a steel corrosion inhibitor for petroleum productionapplications in mineralized media with high O2 content. Thecompound is manufactured by condensation of zinc chloridecontaining o-aminophenol with an oil aldehyde in benzenefollowed by reaction with zinc chloride [36]. Acosta et al.patented novel piperidine/piperidinium compounds ascorrosion inhibitors for use in aqueous medium containingH2S, in petroleum refineries or pipelines [37].Acidic fluids (HCl, HF etc) are often used as a treatingfluid in wells penetrating subterranean formations for clean-up operations or stimulation operations for oil and gas wells.Corrosion inhibitor intensifiers have been used to extend theperformance range of a selected acid corrosion inhibitor.Most intensifiers such as KI, Sb-based compounds etc havetemperature, time, and environmental drawbacks. Wilson etal. disclosed 2-chloro-2,2-diphenylacetic acid and 2-bromo-isobutyric acid as corrosion inhibitor intensifiers [38].Beloglazov et al. disclosed an antipyrine derivative as aninhibitor against microbilogical (myxromycete) corrosion ofequipment made of carbon steel and alloy steel withcadmium coating [39].Usually in hydrocarbon pipelines, inhibitor is injectedinto the stream which causes the inhibitor to stay with theliquid at the bottom of the line, making the top line morevulnerable to corrosion. In a recent patent, corrosioninhibitor was applied within a foam matrix to the top of theline of hydrocarbon pipelines made of carbon steel. A stablefoam matrix was created by the combination of a foamingagent, foaming gas and the corrosion inhibitor [40].CORROSION INHIBITORS IN PAINT ANDCOATINGSCorrosion protection by coatings can be passive (onlybarrier effect) or active (corrosion inhibitors are incorporatedin coatings) or a combination of both. An industrial coatingsystem (for e.g. aircraft coating) usually consists of threeindividual layers; the conversion coating/anodization layer, asecond primer layer (pigmented organic resin matrix-principal provider of corrosion resistance), and the toppolyurethane/polyol resin coat (principal barrier coating)[41,42]. Significant efforts are currently underway in findingalternatives to chromate conversion coatings (due totoxicity). Addition of inhibitors in sol-gel coatings (whichwere projected as substitutes to chromate conversioncoating) often results in detrimental properties. Theinhibitors incorporated directly into the sol-gel matrix maylose their activity quickly.Inhibitor technology in paints is advanced to greatextents with the help of nanotechnology. Nanostructuredmaterials engineering extends the possibility of engineering‘smart’ coatings that can release corrosion inhibitors ondemand when the coating is breached, stressed or anelectrical or mechanical control signal is applied to thecoating. Smart coatings which can respond to corrosion andthereby inhibit further corrosion are projected as replacementfor chromate based coatings. Research has developedchromate-free corrosion inhibiting additives in whichorganic corrosion inhibitors are anchored to nanoparticleswith high surface areas that can be released on-demand [43].Promising results are expected on nanoparticle/nanostructurecarriers (of corrosion inhibitors) based smart coatings fortheir potential commercialisation. Inherently conductingpolymer films containing inhibiting anions as the dopantanions can release them when the film is coupled to a breachin the coating. However conducting polymers have severallimitations for use in industrial coatings. The film formingproperty of the polymers are not ideal and they have high
  • 4. 66 Recent Patents on Corrosion Science, 2011, Volume 1, No. 1 Viswanathan S. Sajicost and limited solubility. As an alternative, McGee et al.patented a non electrically conducting film formingcopolymer based ‘release on demand’ corrosion inhibitorcomposition. The polymer backbone bears a nitrogencontaining functional group and a metallate anion bonded tonitrogen in the functional group, by ion pairing throughCoulomb attraction. The metallate anion (of Mo, W etc) willrelease from the nitrogen functional group once the pH risesabove a limit, associated with the pKa of the nitrogen group[44].Zhu et al. suggested an alternative to the conversioncoating. By incorporating a small percentage of monomericorganofunctional silanes or oligomeric organofunctionalsilanes into a polymeric coating primer or clear coat, one caneliminate the need for pretreatment. In particular, aminovinyl silane blends are useful in the present invention [45].Gammel et al. patented an anticorrosive paint for metalsubstrates capable of permanent releasing of a corrosioninhibitor. The coating comprises a matrix consisting of aprimer or sol-gel materials containing polymer encapsulatedcorrosion inhibitor such as mercaptobenzothiazole [46].Andreeva et al. patented corrosion inhibiting coating foractive corrosion protection of metal surfaces comprising asandwich-like inhibitor complex and its preparation method.The sandwich-like complex of the corrosion inhibitingcoating comprises a first inner layer of organic species, acorrosion inhibitor layer and a second outer layer of organicspecies. The complex is sensitive to external stimulus (a pHchange) and capable to release the corrosion inhibitor inresponse to the stimulus [47]. Katsiaryna patented acorrosion inhibiting coating comprising a primer layer asfirst layer. The primer layer was incorporated with corrosioninhibitor loaded containers which have the capability torelease a corrosion inhibitor (organic) in response toelectromagnetic radiation. A top second layer having nocontainers prevents spontaneous opening of the containers inthe primer layer induced by electromagnetic radiation. Thecoating is capable of releasing an inhibitor in response toelectromagnetic irradiation if the protective top layerdevelops defective areas which enable direct exposition ofthe containers to the electromagnetic radiation. The containerhas photoactive anatase titania core. The inhibitor is presentin pores and the core is enclosed in a polymer shell. Thepatent made use of photocatalytically active porous materials(anatase titania) which act as initiators for the release ofencapsulated inhibitor only when they are exposed toelectromagnetc radiation in damaged areas [48]. Golovin etal. patened a method of protecting polymer coated metalfrom corrosion using inhibitor microcapsules. The polymercoating composition contains microcapsules with corrosioninhibitor with a polymer shell. Microcapsules with thecorrosion inhibitor are made of an inert sorbent (porousalumina, porus silica etc) with polymer shell into pores of whichthe inhibitor is impregnated. The method improves corrosionprotection of coated metal surface due to continuous release ofinhibitor during operation of coating [49].Chromate-free pigments available today for paints andcoatings are typically phosphates, silicates, borates,molybdates or cyanamides of Zn, Ca, Sr, Al, Ba, Mg, or Ce,and in many cases are mixtures thereof. Many patents areavailable on novel inhibitors/inhibitor combinations forapplication in paints and coatings. Boocock patented a non-chromate and highly crystalline hydrotalcite inhibitor.Hydrotalcite, containing the decavanadate ion is the sourceof corrosion inhibition and is produced by high temperaturereaction of a mixture of zinc oxides/hydroxides withaluminum hydroxides and vanadium oxide. The compositionis highly effective in providing blister-free corrosionprevention in typical coil and aerospace grade epoxy primerand color coat combinations, to prevent corrosion of Al orhot-dip galvanized or galvalume coil stocks [50]. Slaghek etal. patented chemically modified polysaccharides ascorrosion inhibitors and/or preservatives to a compositionsuitable for coating solid surfaces [51]. A corrosion inhibitorformula based on crystalline permanganate soldalitecompositions was patented for pigment applications [52].Tayler et al. explored synergistic inhibition of anodic (fromvanadates, molybdates, tungstates, phosphates, borates) andcathodic inhibitors (from Ce, Y, La, Eu, Gd, and Nd) assuitable substitute for chromates [53]. Monzyk et al.patented metal ferrate compounds as inhibiting additives inprimer coatings employing novel method which increasedferrate compounds solubility in water [54]. Jaworowski et al.patented an anodic-cathodic corrosion inhibitor-conductivepolymer composite. The patent provided a conductivepolymer corrosion protective coating to be applied tostructures such as turbine engine components which includesa conductive polymer with corrosion inhibitive pigments oradditives comprising an anodic (vanadate, molybdate,tungstate and/or mixture) and a cathodic (cerium,neodymium, praseodymium, and/or mixture) corrosioninhibitor. The hybrid composites can be used in anti-electrostatic, electrostatic dispersion and electromagneticshielding applications [55]. Mohammed explored organic-inorganic composite resin coatings as alternatives tochromate coating, wherein the corrosion inhibiting additivesare zinc acetylacetonate and benzimidazole derivative [56].A corrosion inhibitor in combination with a metal ionscavenger, e.g., a metal chelating agent can act as a quantumdot quenching-preventive agent. When quantum dots areapplied to metals such as Cu, brass, stainless steel, bronze,Al, Ni etc, the fluorescence intensity of the quantum dots canbe dramatically reduced or quenched. In addition, thequantum dots may easily rub off of the substrate materials,particularly smooth materials such as stainless steel andbrass. This limits the current use of quantum dots on suchmetallic surfaces. The respective amounts of the corrosioninhibitor and the metal ion scavenger can be optimized toprevent the quenching of the quantum dots in the presence ofthe metal ions [57].ACID CORROSION INHIBITORSInhibitors for acid corrosion, especially for Fe and steelhave widespread use in different industries (e.g. oil and gasfield). Mostly organic inhibitors are employed as acidinhibitors; as the passivating inorganic inhibitors may bedangerous in acid environments and cause severe localizedattack once the passive film get broken. This sectiondescribes recently patented acid corrosion inhibitors forapplications such as in acid pickling and acid cleaning.Khomyakova et al. patented a chemical composition foruse in etching of metals and acid treatment of equipment in
  • 5. Contemporary Developments in Corrosion Inhibitors Recent Patents on Corrosion Science, 2011, Volume 1. No. 1 67energetics, food industry etc, that comprises of n-bromine-benzal-m-nitroaniline, 2-chlorine-6-diethylamino-4-methylpyridine, 1,3-bis(carbamoyltio)-2-(N,N-diemethylamine) propanehydrochloride, and urotropin. The composition is good forprotection of steel, Al and Ni, and also for reduction of steelhydrogenation [58]. Kurochkin et al. patented an inhibitorcombination of 5-nitrosalycylalsulphathiazole, 3-dodecyl-benzimidazole iodide, polyethylene-polyamine; for protect-ion of steel, Ti and Al and also for inhibition of hydrogen-ation [59]. A combination of -oxynaphtalisonicotine hydra-zide, 2,4,6-tris(2-isotioureido)-s-triazine hydroiodide, 2-(tiazolyl-4)-benzimidazole and urotropin is disclosed byKravtsov et al., for steel, Al and In and for reduction ofhydrogen charging of steel [60]. Alkarylated polyalkylpyridinium salts inhibit over pickling of Fe and alloys inHCl/H2SO4 pickling baths [61]. Li et al. proposed anenvironment friendly inhibitor used in pickling of carbonsteel. The inhibitor contains 1-amino-2-mercapto-5-[1 -(1 ,2 ,4 -triazole)-methylene]-1H-1,3,4-triazole and/or 1-phe-nyl-2-(5-[1 ,2 ,3 ,5 -tetrazole-methylene]-1,3,4-furodiazole) thio-alkyl ethyl ketone [62]. Yafarova et al. patented an inhibitorcomposition of n-dipentyl-amine-salicylal-2-amino-4-nitro-phenol, (indolyl-3)acetic acid, 3-ethyl-2[5-(3-ethyl-benzo-thiazolinyliden-2")pentadien-1,3-yl]benzo-thiazoliyiodide,and urotropin for steel, Ti and In [63].It has been discovered that simple diols (ethylene glycolor propylene glycol) and triols (glycerol) work as corrosioninhibitors in concentrated organic and inorganic acidswithout any neutralization of the acids. These corrosioninhibitors are cheap and readily available in large quantities.The diols and triols also reduce evaporation of acid, and thusimprove the working environment [64].Phosphoric acid is a preferable choice for acid cleanersdue to low cost, low corrosivity and ability to exert a verystrong synergistic cleaning affect. However, phosphoric acidis not a good choice in terms of environmental concerns.Miralles disclosed a phosphate free acid cleaningcomposition: an aqueous urea sulfate based acid cleaningcomposition having gluconic acid as corrosion inhibitor forstainless steel. The combination of gluconic acid in acidiccleaning solutions works well and inhibited all corrosion.Any traditionally used acids (such as acetic acid, citric acid,oxalic acid, sulfuric acid etc) in cleaning compositions canbe combined with gluconic acid. The acid cleaningcomposition is biodegradable, less expensive and retains theanti-corrosive properties of phosphoric acid as well as thecleaning capabilities [65].CORROSION INHIBITORS IN LUBRICANTS,HYDRAULIC AND METAL WORKING FLUIDSOne of the causes of wear in an internal combustionengine is corrosion of the metal surfaces of the engine. Thecorrosion promoting compounds present in the crankcase areprincipally weak organic acids which may result fromnitration and oxidation of the lubricating oil due tocontamination by blow-by gases and exposure of thelubricant to high temperatures. For the purpose of preventingcorrosivity by these compounds on the various engine parts,it is necessary to incorporate dispersants, detergents, andcorrosion inhibitors in the lubricating oil composition. It isdesirable to minimize the amount of phosphorus inlubricants. Although phosphorus does not contribute to ash,it can lead to poisoning of catalysts.Boudreau disclosed a lubricant composition containingphosphorous and sulfur free organotungstates impartingimproved antiwear, corrosion, and antioxidant properties.The organic tungsten complex is a reaction product of amono- or diglyceride and a tungsten source, or of a fatty acidderivative and a tungsten source [66]. Corrosion inhibitorssuch as oil soluble 2,5-dimercapto-l,3,4-thiadiazole andhydrocarbyl-substituted 2,5-dimercapto-l,3,4-thiadiazole deri-vative was used in a patented lubricating composition byTipton et al. [67]. Sodium or calcium salt of dinonyl-naphthalenesulfonic acid was used as a corrosion inhibitor invegetable oil based lubricants [68]. A patented phosphorousfree additive composition contains a hydrocarbyl-substituted1,2,4-triaxole corrosion inhibitor [69]. Ivanov et al. disclosedan inhibitor composition as additives into motor oils andlubricating cooling liquids that contains a lanthanum oxide,triglycerides of higher carboxylic acids, alkylbenzene-sulfonic acid, alkanolamine, lanthanum nitrate and organicsolvent in the balance [70].VAPOR PHASE CORROSION INHIBITORSVapor phase (volatile) corrosion inhibitors (VCIs) havethe particular advantage that the vaporized molecules canreach hard-to-reach areas commonly found in electronicenclosures, between two metal flanges and similar othersystems. The VCI envelop the metal article in a non-corrosive layer and retard moisture and oxygen present in theatmosphere from attacking and reacting with the metalsurfaces. VCI may be applied by combining the inhibitorwith a liquid and spraying the entire surface of the metalarticle to be protected. Alternatively, VCI may beincorporated into a packaging material such as paper andplastic wraps, films, and plastic dunnage for the protection ofmetals from corrosive environments during for exampleshipment, storage or handling. Different VCIs orcombinations of VCIs may be selected based on the type ofmetal to be protected, the size of the enclosure, and thelength of time that protection is required.Most of the patents in this area are dealing with novelproducts using the conventional VCIs. A recently patentedVCI containing packaging films for metals includesphosphate esters, caprylic and isononanoic acid and amino-2-methyl-1-propanal [71]. Damiani patented an improvedskate guard comprising a pouch and a corrosion inhibitorwhich is useful for protecting the blade of an ice skate duringstorage and protects the blade of the skate from rusting. Asilicone based VCI was employed for the same [72]. Morotaet al. patented an anticorrosive tri-layer polymer sheetcomprising a propylene intermediate layer containing VCI.The steel sheet (JIS G 3141) stored in the polymer containerat 50 oC and relative humidity of 100% for 14 days exhibitedcomplete protection [73]. Lyublinski et al. disclosed a rust-resistant tape that comprises a polymer base material layer,an adhesive layer and a rust-resistant component. The rust-resistant component comprises at least one VCI that isplaced into the matrix of the polymer base material or theadhesive layer. The rust-resistant tape can be directly applied tothe metal surface to be protected [74]. A synergistic corrosionmanagement system designed to mitigate corrosion in
  • 6. 68 Recent Patents on Corrosion Science, 2011, Volume 1, No. 1 Viswanathan S. Sajicontainers, enclosures, cisterns and/or storage tanks utilized acombination of cathodic-based corrosion prevention system,soluble corrosion inhibitor, and a VCI [75]. Daisuke patented anovel corrosion preventive packaging bag [76]. Chinn et al.disclosed a dual layer corrosion barrier system, a corrosionbarrier of VCIs and an anti-wetting barrier having a nanoparticlecomposite structure (having nonconductive particles of alumina/silica) [77].Very high volatility of VCIs is a disadvantage when thesecompounds are incorporated into organic polymeric materials athigh temperatures. Additionally, VCIs must have a high thermalstability. The known VCIs may not satisfy in every respect thehigh requirements which a corrosion inhibitor is required tomeet, especially with regard to incorporation into plastics athigh temperature. It has been found that alkylaminosiloxanesbased compounds are particularly suitable for use as corrosioninhibitors for protecting metallic surfaces in such applications[78].Chen et al. patented a green slow-release metal VCI. TheVCI was incorporated in a -cyclodextrin. The -cyclodextrinwas first dissolved in water and to the saturated solution, VCIwas added followed by inclusion reaction and freeze drying.The slow-release VCI can be used directly for producing anti-rust paper and plastic antirust film. The slow-release VCI haslong acting effect, improved stability and heat resistance [79].One of the issues with current VCI emitting packagingmaterials is that the inhibitor often can not be seen or detectedby close inspection. This creates a number of drawbacks to theuse of VCIs. For example, end users of the packaging article donot have a convenient method to confirm that the packagingarticle contains an appropriate VCI mixture. Unless expensiveanalytical tests are performed, it is difficult to assure that thevendor incorporated the VCI concentrate or master batch intothe articles in the right amounts. McConnell disclosed acorrosion inhibiting mixture comprising a carrier, a VCI, and atracing agent which absorbs light in the ultraviolet region of theelectromagnetic spectrum, and re-emits light in the blue regionof the electromagnetic spectrum. Particularly significant in thisregard is the potential the invention affords for providing a highquality, low cost VCI mixture tracing agent [80].CORROSION INHIBITORS IN OTHER MAJORAPPLICATIONSCorrosion Inhibitors in Electronic IndustryChemical mechanical polishing or planarization (CMP) isa process in which material is removed from a surface of amicroelectronic device wafer, and the surface is planarized.In its most rudimentary form, CMP involves applying slurry,e.g., a solution of an abrasive and an active chemistry, to apolishing pad that buffs the surface of a microelectronicdevice wafer to achieve the removal, planarization, andpolishing processes. Many patents are available on CMPsolution utilizing corrosion inhibitors [81-83]. Yoshikawa etal. patented a composition for mechanochemical polishingthat was comprised of phosphonium salt, triazole basedcorrosion inhibitor and colloidal silica [84]. Chang et al.patented sarcosine and its salts as corrosion inhibitors duringcleaning after chemical mechanical polishing [85].A patented printed circuit board with long term reliabilityconsists of corrosion inhibiting layers containingbenzotriazole or nitrophenylhydrazine [86]. Novel imidazolecompounds were patented for surface treatment of Cu or Cualloys in printed circuit board manufacture [87]. Huang et al.patented an antistatic antirust film (polyolefin resin andantistatic agents based) containing volatile corrosioninhibitors (monoethanolamine benzoate, sodium molybdate,2-ethylimidazoline). The film is applicable in rust preventionof steel, Cu or Al and does not affect nonmetallic materials[88]. Zhou and Wu patented a metal surface protecting agent(organic corrosion inhibitor, film forming component,surfactant, organic solvent) for protecting Ni plating layer,conductive oxidation layer of Al alloy, Au plating layer,stainless steel, Cu and Zn plating layers [89].Electronic devices comprising of Cu/Cu alloy substratesare typically coated with precious metal coatings (Ag or Au)which provide corrosion protection, high surface contactconductivity and wear resistance. Due to the cost factor, theindustry typically employs precious metal layer on the orderof about few micrometer thickness on electronic connectors.However, these thin layers having a high degree of porositymay become ineffective and cause Cu diffusion to thesurface. The resulting corrosion products can migrate ontothe coating surface and deteriorate the surface contactconductivity. Abys et al. disclosed a ‘method and composit-ion’ for enhancing corrosion protection, solderability andwear resistance of Cu substrates used in the manufacture ofelectronic and microelectronic devices. The methodcomprises exposure of the Cu alloy substrate having theprecious metallic surface layer, to an aqueous compositionthat effectively forms self assembled monolayer on both theprecious metal and the Cu surface. The invention made useof self assembled monolayer formation of thiols on preciousmetal surfaces. An amine based organic molecule was usedto interact with and protects Cu surfaces [90].Concrete Corrosion InhibitorsTobacco products such as dried tobacco leaves, stemsand dust are added to cementations concrete and mortar toinhibit corrosion. The tobacco addition protects steelembedded in Portland cement from corrosive attack.Tobacco is renewable, potentially inexpensive bioproductthat provides excellent corrosion protection with little or noenvironmental concerns [91]. Imasawa patented an aqueousstyrene-butadiene co-polymer emulsion corrosion inhibitorfor reinforcing steel for autoclaved lightweight concrete [92].Calcium nitrite and surfactant based composition waspatented by Takeshi and Hideo [93]. A 50:50 combinationsodium benzoate and alkali metal sebacate was used as amigrating inhibitor (VCI) in the polymer fiber, in post-tensioning cables [94].Other ApplicationsNanoscale inorganic corrosion inhibitors such asnanoscale silica, nanoscale titania, nanoscale zinc oxide, ortheir mixtures are used in anti-corrosive antibacterial plasticmetal composite tube. The composite tube comprises aplastic outer layer (contains 0.2-8% of inhibitor), a plastic
  • 7. Contemporary Developments in Corrosion Inhibitors Recent Patents on Corrosion Science, 2011, Volume 1. No. 1 69inner layer (contains nano Ag), and a metal layer. Theinventive plastic metal composite tube has the advantage ofgood sterilization function on water body, good resistance toaging, long service life and no pollution [95].Ureido silanes are patented as aqueous corrosioninhibitors [96]. Quraishi et al. disclosed a process forpreparation of a stable 1-cinnamylidine-3-thiocarbohydra-zide, the compound acts as a corrosion inhibitor for mildsteel [97]. Biodegradable and non-toxic sodium triethyl-enetetramine bisdithiocarbamate was patented for corrosioninhibition for Cu in acidic conditions [98].Many other patents are reported on corrosion inhibitorsfor various applications such as in batteries [99, 100], inelectroless plating [101], in washing detergent composition[102,103], in road deicing applications [104,105], in coatingremover compositions [106] etc. A trace amount of tungstateadded together with an ortho-phosphate and/or phosphoniteis disclosed for inhibiting corrosion by an ammonium nitratefertilizer solution in contact with ferrous metal storage tanksand equipment surfaces [107].CURRENT & FUTURE DEVELOPMENTSR&D in different areas of corrosion inhibitors isexploring newer methods and products. Conventionalinorganic inhibitors continue to be a major component inmany patented inhibitor combinations. Few novel inhibitorsare introduced. Many of the reports deal with novelinhibition methods in various applications. Few patentsexplored nanotechnology for better inhibition. Interestingpatents were reported on smart coatings that can releasecorrosion inhibitors on demand. Novel packaging productsare patented based on VCIs. The trend indicates that “ondemand releasable” corrosion inhibitors are going to be themajor area of interest. This is especially relevant indeveloping smart coatings as replacement to chromateconversion coatings. Suitably modified nanoparticle/nano-structured carriers are effective for high loading of organicinhibitors. Synergism in inhibition between non chromatepassivating inhibitors with biodegradable organic inhibitorsneeds to be explored.ACKNOWLEDGEMENTThis work was supported by World Class UniversityProgram at UNIST.CONFLICT OF INTERESTThe author declares no conflict of interest.DISCLOSUREPart of information included in this article has been prev-iously published in Recent Patents on Corrosion Science, 2010,Vol. 2, pp. 6-12 (Open Access Journal).REFERENCES[1] Nathan CC. 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