Paper No.

232

CORROWOIUC)LThe NACE International Annual Conference and Exposition

Corrosion Inhibitors in Fertilizer Production and Handling

Prof. G.V. Prabhu-GaurtkarDept. of Metallurgical Engineering and Materials Science

Indian Institute of TechnologyBombay-400076, India

Prof A. RamanMaterials Group, Dept. of Mechanical Engineering

Louisiana State UniversityBaton Rouge, LA 70808 USA

ABSTRACT

In fertilizer production, storage and handling, corrosion inhibitors play a very important role in ensuring thedurability of the equipment and plant structures. The plants handle very corrosive raw materials and several corrosiveintermediates and by-products under operating conditions. Fluid velocity and high temperatures and pressures compoundthe corrosion problems. In this paper an attempt is made to review the materials commonly utilized in the fertilizer plantsand the role of inhibitors in mitigating various corrosion problems encounter~ based on the chemicals commonly handledand the products generated and distributed. Corrosion inhibitors for liquid ammonium nitrate-urea fertilizer are described indetail.

Keywords: Fertilizer compositions, SCC inhibitors for ammonia, acid inhibitors, liquid ammonium nitrate-urea fertilizer,phosphate esters, amine oxides.

INTRODUCTION

As in many other industries, corrosion inhibitors (CIS) are also used in the fertilizer industry to protect equipmentand structures from corrosion. The problem of protecting the equipment used in storing and dispersing the fertilizers alsoimpacts the ultimate customer. The strategy adopted in ensuring durability and safety of installations and equipmentinvolves a judicious compromise between the selection of fully corrosion resistant materials and the utilization of relatively

Copyright@l 996 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole must be made in writing to NACEInternational, Conferences Division, PO. Box 218340, Houston, Texas 77218-8340. The material presented and the views expressed in thispaper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.

less corrosion resistant materials with adequate protective measures. The latter generally involve cathodic protection,

protective coatings and inhibitor systems. CIS may sometimes be ignored by the plant persomel due to the unfamiliarity

with the chemicals, or processes involved. Ordy in cases where the inhibitor application is the only viable corrosion

protection method, is the prescription followed earnestly.

During fertilizer production, virtually all kinds of corrosion problems are encountered14, under the prevalentconditions of elevated temperatures and pressures, the presence of very corrosive chemicals and the by-products handled,high fluid velocities, and abrasion, erosion, etc. Though all the corrosion problems cannot be solved using inhibitors alone,many of the CIS used in other industries can be used in the fertilizer industry as welL5. To combat stress corrosion, erosion-corrosion, cavitation, etc. industrial problems by applying inhibitors alone is ditlicult, and practical exqxxience needs to berelied upon heavily. For normal corrosive situations, recommendations for the optimum choice of materials for handlingdifferent products, with or without additional corrosion protection measures, are available in the literature,&9 along withthe specifications for optimum inhibitor systems. Since the raw materials utilized and the products generated and handledare known, the problem of selection of the inhibitor system may appear to be simple and straight-fonvard. However, inpractice, the choice and application of the proper CI system is not that simple, for the working conditions are severe andvaried. A brief summary of the raw chemicals and the fertilizer products handled and the materials of constructionutilized 10>1is presented first. A review of the CIS used at various stages of fertilizer production is presented further.Experiences reported on the effectiveness of difTerent types of CI formulations in the fertilizer industry are discussed and anattempt is being made in this paper to ident@ the areas where the problems still persist and how further research anddevelopment might stimulate the development of new inhibitor systems for application in the field of fertilizer systems.

Fertilizers and related chemicals:

The three principal constituents of fertilizers are: N - nitrogen (NHs), P - phosphorus (PZOS),and K -potash (K~O).Commercial fertilizer mixlures contain these primary constituents in varying proportions and many may also contain someminor constituents such as sulfur, sodium, calcium, magnesium, borou iron, manganese, zinc, etc.11 Some important typesof fertilizers are listed in Table 1. Typical requirements in terms of chemicals and raw materials used for producing one tonof a common fertilizer grade are given in Table 2.

The principal chemicals required are generally manufactured in the fertilizer plant itself. In addition, sulftic acid,naphtha and natural gas, steam, etc., are utilized in the production prccess. The actual corrosion problems depend on thechemicals used and also on the prwess parameters. The inhibitors used are generally those used elsewhere, but specialconsiderations have to be given to their functional mechanisms under the conditions prevalent in fertilizer production andhandling. Salient CI systems used in various locations of a typical fertilizer plant are summarized in Table 3.

Equipment and the materials of construction:

Equipment used for the production and handling of fertilizers and related products generally consists of differenttypes of reactors, Imilers, scrubbers, adsorption and drying towers, storage tanks, heat exchangers and cooling towers,pipelines, pumps, compressors, fans, valves, conveyors, etc., along with the structures supporting these units. Carbon steels,low alloy steels and cast irons are amongst the cheapest materials often used along with suitable corrosion protectionmeasures. Special steels and stainless steels and corrosion-resistant nickel alloys are used for some highly corrosivesituations. Development and use of specially engineered alloys for the fertilizer industry is an on-going process. 1112Some of the most expensive materials are used in components of pumps, valves, etc. without the use of any corrosionprotection measures.13>14In addition, the atmosphere in fertilizer plants is usually highly aggressive due to leakage ofprocess chemicals, pollutants, etc., which gives rise to serious corrosion problems of supporting structures. These aregenerally protected by special coating formulations containing inhibitors.]s

Role of Corrosion Inhibitors

The role of CIS can be examined in relation to the chemicals involv~ the fertilizer products generated and thecorrosion problems encountered during the process. Special consideration has to be given to the evaluation of theirmechanisms of action and efficiencies under the prevalent process parameters. Finally, the CIS utilized in storage andhandling of the finished products should be considered.

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The fertilizers and related chemicals may be grouped as follows:

a) ammonia and ammonium compoundsb) fertilizer acidsc) other products and mhdures

The CIS utilized in each category are discussed below.

Ammonia and ammonium compounds:

Ammonia: Ammonia is used as a direct fertilizer and as a chemical intermediate in the synthesis of other nitrogen-containing compounds. Materials handling ammonia sometimes have problems due to decarburizatio~ nitriding,embrittlement due to hydrogen, cracking, etc., which are taken care of by selecting and using appropriate corrosion resistantmaterials, control of process parameters and suitable inspection and replacement schedules. G*7 The major problemsrequiring application of CIS are encountered in the storage and transport of ammonia. Incidents involving fatalities havebeen reported due to corrosion failure of ammonia tramprt vessels. ]s>lgThese failures have been attributed to stresscorrosion cracking (SCC) of the steel tanks in the ammonia medium.20 Ammonia is handled under varied conditions, fromfully refrigerated to partially refrigerated, or unrefrigerated (under ambient temperature and pressure). The US Departmentof Transportation recommends safeguards for the ammonia tanks made of steel in the quenched and tempered condition,which include a post weld heat treatment and the use of 0.2/0 water as additive to inhibit the stress corrosion cracking.2021It was recognized in the fifties that cdcm steel is prone to SCC in concentrated ammonia.22 Longimov and Phelps3carried out investigations on the factors effecting SCC in liquid ammonia and established the role of water as the inhibitorin this case. Several others have investigated the SCC of steel used in the storage of liquid ammonia.24-30 It has beenestablished that presence of oxygen and carbon dioxide is essential for the cracking to take place. Nitrogen contaminationincreases the susceptibility to cracking further.

Wilde31carried out accelerated tests on ASTM A 517- grade F steel in liquid ammonia and found that oxygencontamination is the primary cause for the SCC and that the simultaneous presence of nitrogen increases markedly thesusceptibility to the SCC. He firther showed that as little as 0.01 ppm of oxygen could cause SCC, and water additions aslow as 0,08 wt.Yocan effectively inhibit SCC in the ammonia environment containing up to 200 ppm oxygen. Hydrazineaddition at 0.0250/0 was found to be effective in supplementing the role of water and mitigating the SCC.32 Water additionsat 2000 ppm inhibits the SCC of steels in air-contaminated liquid ammonia solutions at OC.2 Hallan33 has reported that 3ppm of oxygen could initiate the SCC in high strength steels at -35C. Proctor et al.34 have shown that additions of 300 ppm

of water prevented the SCC completely. Lunde and Nybor~ found the highest susceptibility to cracking when 3-10 ppmoxygen was present along with insutllcient amount of water, at

Overall reaction: 2 Fe + Oz + 2NH4+= 2 Feq++ 2 OH- + 2 NHs (3)

The presence of C02 along with air is said to give rise to ammonium carbamate:

2 NH3 + co~ = NH4c02NH~ , (4)(with HZOaddition, (NH4)ZCOS,ammonium carbonate could form) (4A)

which would increase the ammonium-ion concentration:

NH4C0,NH2=NH4++NH2c02-((NH4)2C03 = ZNH4+ + CO/- )

(5)(5A)

In ammonia vessels, the cracking starts as transverse cracks in plates and when these have developed sutliciently innumber and size, longitudinal cracks begin to deveIop.31In laborato~ tests the cracks were found to be transgranular andcleavage-like34, involving rnicrocleavage. The crack growth occurs by the anodic thick film rupture mechanism, ratherthan by the slip-step dissolution process. Wilde31, however, has propmed that film rupture at emergent slip steps is thebasic mechanism. Adsorbed nitrogen atoms at the crack tip would impede repassivation and thus promote unabated crackgrowth, but water counteracts this tendency. It would repassivate the crack tip before the nitrogen atoms get adsorbed atthese sites.3G

In a basic study dealing with the corrosion of many metals in liquid ammonia, Jones and Wilde3Gfound thataluminum, when connected to steel, acts as a saeritlcial anode and protects the steel from SCC in air-contaminated liquidammonia. This implies that cathodic protection from SCC is feasible in this environment also.

Corrosion inhibitors function basically by forming protective films, or by reacting with the corrosive species in theenvironment. Water is reported to promote passivity of steels in the ammonia-methanol system.34The protective film,according to these authors, is not an oxide film but might be iron nitride. Ahrens et al.32also have suggested the formationof iron nitride film on the steel in the ammonia environment. Proctor et al.34have fiuther expounded that the nitrogen-containing film FqN gives rise to a film-induced discontinuous cleavage nucleation. Water acts to stabilize the protectivefilm and prevent the cleavage nucleation.

In spite of the understanding gained on the mechanisms of inhibition by water, the problem of SCC still seems topersist. This is primarily because of the ingress of oxygen in large amounts, which cannot be completely controlled andeliminated in practice. The water available for reaction is present in relatively low concentrations and it also gets depleteddue to the reaction with the crack walls.

Additions of ammonium chloride or a suitable oil have been reported to retard SCC in liquid ammonia.37 Anotherproposition calls for adding 10 ppm of vegetable oil as an effective inhibitor.38 Though 0.2% water has been effectively usedfor inhibiting SCC of steel in liquid ammoni% problems still persist and attempts to ke.tter understand the inhibitionmechanisms3Gand to develop new and better inhibitor systems continue. Diffkulties have been reported in repairs andprevention of new cracks from growing.3g40Dynamic water infusions to slow down or arrest the propagation of activelygrowing cracks have been suggested by Proctor et al.w Some data on crack growth rates in steel in ammonia environmentsis available. It will be useful to generate adequate data on Km and crack growth rates on the steels used in the ammoniaenvironments under various conditions of temperature and pressure. Also, the role of water needs to be elucidated further..Suitable in-service inspection procedures would help in verifying the eff~tiveness of the inhibitors333g40and in ensuringthe integrity of the ammonia storage and handling vessels. Efkctive methods of rust prevention in ammonia storage tanksshould be developed which would help the inhibition.

Condensation of ammonia vapors on the top walls of the tanks can produce SCC and one of the practical solutionssuggested33to solve this problem is to maintain the temperature of liquid ammonia a few degrees lower than thetemperature of the walls, where the condensation of ammonia vapor would occur.

Kain et al.41 have established the application of O.15% K2Cr207 as CI for carbon steel in contact with 17-27%.ammonia at pressures up to 15 kgjcmz. However, direct reduction of bichromate and formation of chromate (C/+-ions) inthe ammonia streams lead to erosion-corrosion and related failure. In view of the toxicity of chromate salts, the applicationof this procedure requires a lot of careful consideration.

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Ammonium compounds: Fertilizer compounds which can decompose producing ammonia are very aggressivesubstances. Compounds such as dihydrogen ammonium phosphate or ammonium nitrate can lead to increased corrosion dueto hydrolysis to acids, thereby giving rise to a reduction in the pH and highly acidic conditions. Ammonium nitrate and ureareact slowly with steel in general, but can act rapidly at stressed locations such as welds, bolt holes, etc.42 Ammoniumnitrate and phosphates are hydroscopic and become corrosive by absorbing moisture. These are to be handled underconditions of controlled humidity in order to control corrosion. Sulfur compounds such as thiourea, ammonium thiocyanate,and 2-mercaptobenzothiazole are used as CIS for mild steel exposed to ammonialammoniurn nitrate/water system. Insystems containing also urea, these sulfur-compounds are not very effective. Orthophosphates are also used as inhibitors,but free ammonia (contributing to corrosion) would reduce the protection offered to the exposed metal.4344

Phosphate esters are found to be good CIS for nitrogen-containing fertilizers. Schapira et al.45have patented (RO)P(0)(OR)OH and (RO)P(0)(OH)Z (where ~ R = heptyl, octyl, nonyl, decyl, etc.) and their alkalinemetal salts as suitable CIS. These are prepared by reacting P205with correspondingalcohols. Similar ester compositionswith the organic radicalsbeing Cl.1!3alkyl, alkcnyl,alkynyl, aryl, etc. also havebeen patentedby Isc and Nogawa46in Japan.Polyethoxylatedphosphateesters @olyethoxylatedNa-phenylphosphateand polyetho~lated Na-triphenylphosphate)aregoodCISfor NH4NOJ- urea fertilizer.The latter CI at 500 ppm concentrationgivesthe best inhibition efficiency.47

Doll et al.4* have patented an inhibitor mix containing alkyl phosphates for liquid nitrogen-fertilizers. Hisformulation consists of A@gO~o mono-, or di- alkyl phosphate, containing CGIOalkyl chains, 5-30A alkox@ated compounds,5-30% polyalent aliphatic acids and/or polyalcohols. Another patent49 suggests the use of 0.002A by WI. of (NHg)2HP04 in50% dicyandiamide as CI for steels in liquid ammonia fertilizer.

For liquid ammonium nitrate-urea fertilizer, 30-95%.alkyl phosphoric acid monoester ammonium salt, with 3-20V0C,7.24fatty acid sulfate and 3-25% Na-soap RCH(COzNa)OSO~Na (R = C,wzOalkyl) is recommended.50 Doll et al.s havepatented a mix containing 40-90 mono-, and di- C61Oalkyl phosphates, 3-20Y0 alkylated mono-, and di- alkyl ammoniumphosphate, 5-40A CIo13 alkyl benzene sulfonates and 3-20%. aliphatic alcohols and/or polyalcohols or their derivatives asCI for steels in the above media. The same authors also are advocating in another patent52 alkyl sulfonylglycines M (CHJnS02 NHCH2C02R (R = H, NIL, M, M= alkaline metal; n=8-24) as CIS for similar application. A Czech patent by Balena etal.53 simply suggests the use of partly concentrated sulfite liquor for liquid ammonium nitrate as well as for its mix withurea.

Miller and Blaschke54 have been granted a patent in USA in which they suggest the use of amine oxides as CIS forsteels in the ammonium nitrate-urea liquid fertilizer. RR R2N0 RR1 ,R2 = Cl .24alkyl; C&24aryl or alkyl-aryl) type aminooxides are usefti. 300 ppm decyl amine oxide is said to give total inhibition to steel corrosion in a solution containing 40/0ammonium nitrate, SsO/o urea and 25/0 WIkr.

Gru et al.55 describe in a patent that the liquid ammonium nitrate-urea fertilizers are stabilized for long timestorage, and to avoid decomposition, by using 3(5)- methyl pyrazole, dimethyl pyrazole or methyl pyrazole at 0.5-5/0 level.In 20%.urea solution, NaNOZ - NazC03 and NaNOz -ZnSOJ mixes have been studied as CIS to prevent steel corrosion andfound to be effective.5s Thus, inorganic CIS also can find their application in the fertilizer field. On the contrary, for Alcorroding in NaN02 solution at 30C, thiourea and substituted thioureas act as CIS.57 These compounds get adsorbed freelyon the Al-surface and fimction as cathodic inhibitors. Their high energy of adsorption aids in forming surface films that arestable and protective.

For carbon steels in aqueous sulfur dispersions, applied as secondary fertilizers, Mg citrate solution containing 1.2mole NH3 /g-atom of Mg with a pH of 9.7 is a good inhibitor. This inhibitor is applicable in the soil in which the steelcomponent, say a tank or a steel pipe, is buried, in order to protect it from corrosion due to water containing stray sulfiudispersions.58

A book on corrosion inhibitor application and other temporary protection measures5g to protect steel and othermetals from corrosion from fertilizers is stated to be available in the Polish language, but neither this book nor itstranslation is accessible to the authors of this paper. As suck no data is presented from this source.

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Fertilizer acids:

Fertilizer plants produce and handle acids, viz., phosphoric acid (PA), sulfiuic acid (SA) and nitric acid (NA). Thehandling of such highly corrosive acids requires special corrosion-resistant materials, which would be used with or withoutadditional corrosion control measures.

Phosphoric acid (PA): Most of the PA used in the fertilizer industry is produced by the wet process, whichgenerally consists of reacting phosphate rock concentrate with cone. sulfuric acid to form Ca sulfate dihydrate crystals andfiltrate containing PA, which may be separated by solvent extraction and concentrated fimther to be used to make thephosphate fertilizers. The wet process PA contains impurities such as Cl-, F, Few, SiOJ2-, etc., depending on the source ofthe phosphate rock.m>cand the state of concentration of the PA. The process of reaction with SA and sometimesacidulation of the phosphate recks with HC1 adds to these acid impurities in the product. Some of these impurities enhancethe corrosivity of PA and lead to severe plant corrosion problems. The corrosive action of the halides depends upon the PAconcentration and follows the sequence ~Cl>Br for 70/0and Cl>-Br for 300/. acid.cz Moderately concentrated HJP04cannot be handled in unlined carbon steel and becomes corrosive to stainless steels at elevated temperature.c3 Ionizableiodide compounds such as KI,NaI, NH41,etc. are effective inhibitors for mild steel in moderately concentrated PA, but theiodides are not as effective as rubber linings applied to mild steel. Mild steel can also be protected anodically in PA.C3Several classes of compounds such as selenols, thiocyanates, thiols, thiocarbonyls, aldehydes, amines, and inorganicpassivators have been studied for use as inhibitors with the PA.M Nitric acid, which acts as an oxidizing agent, is also saidto help in reducing the corrosion by the PA. One of the formulations proposed for iron corrosion inhibition in PA consists ofthiourea 17., sulfonated castor oil 0.5-15Y0, AsZOS0.5%0,and NasAs040.5% by weight.cs Stainless steels such as Types 316and 317 are used as standard material in dihydrate PA plants, when chloride in the rock is very low, say < 1/0,Higheralloyed material than the conventional stainless steels are required to resist the wet process PAfi storage, transportation andhandling, without any special protection measures being taken. When chloride content of the ores is high, more resistantmaterials, such as duplex stainless steels, nickel-, and cobalt- base alloys, are used.cm

Mechanical abrasion, superimposed on the impurity effects, gives rise to erosion-corrosion problems.cg Corrosionfailures have been reported, particularly in materials serving under highly hydrodynamic conditions, such as pumps, theirimpellers, agitators, etc.70 Pitting and crevice corrosion have been reported as the main causes for failures.

Effects of several impurities in PA on the corrosion of stainless steels and nickel alloys have been studied byseveral authors. bz>ti7075 According to the observations of Linder,fi conventional stainless steels provide useful corrosionresistance over the full concentration range up to about 65C. Use of these steels would be possible up to the boiling pointfor up to 40A concentration. Presence of halogens activates pitting and crevice corrosion. Hart71 has reported particularlysevere problems in evaporator units of wet process plants, where the PA concentration is raised from 45/0 to 7S0/o by wt.Presence of S2- ions would accelerate the anodic processes and alter the passivity of duplex stainless steels used in the wetprocess PA plants. With increased oxygen

stainless steels in PA.T9 This is attributed to the earlier adsorption of Mo in the first stage of the attack.72>T8-81Fertilizergrade PA (737. black acid) attacks steel readily and usually is inhibited with KI. Organic inhibitors are not effective inconcentrated PA when used alone, but it has been reported that a lower concentration of KI is required for inhibition, if afatty acid is also co-added. Ability of the material to develop the passive layer of good quality appears to be the essentialrequirement for the corrosion resistance of steels in the PA enviromnent. Other materials such as At are also used withoxidizing inhibitors, such as sodium bichromate; again, the concentration required depends on the strength of PA.

Sulfuric acid (SA): Concentratedsulfuricacid is used as a basic raw material in the productionof fertilizers such asphosphoricacid, as alreadymentione&diammonium phosphate, triplex superphosphates, etc. Fertilizer plants generallyhave an in-house SA production unit. Weak SA also is used in the production of some of the compound fertilizer grades bythe ammoniophosphate process.

In strong sulfuric acid systems, brick-lined canbon steel is used for absorption and drying towers, pump tanks, etc.,whereas ductile cast iron is used in the piping system.82 Cast irons are susceptible to rapid erosion corrosion and manyproblems due to leakages of cast iron pipelines, particularly at flanges, have been reported. 83Common stainless steels arealso used to handle SA; however, these materials have poor corrosion resistance in hot SA solutions. Stainless steels withhigher Cr and Ni contents and with additions of Mo and sometimes Si have been reported to give superior performance.Duplex stainless steels, nickel and cobalt alloys are used in applications such as impellers, bearing journals, etc., wheresevere erosion corrosion occurs. 834s Failures often result due to localized corrosion.8G>B7These have been attributed to localacid dilutions. Higher than 70/0 concentrations of SA are strongly oxidizing, attack steels slowly and do not requireinhibitors. Dilute SA solutions are also not very corrosive to ferrous materials and can be handled with conventionalaustenitic stainless steel grades. Acids in the intermediate range of concentration are very aggressive and attack iron andnickel alloys rapidly depending upon the temperature, concentration and aeration. These concentrations require specialmaterials and/or corrosion protection measures for handling.

An excellent analysis and discussion of corrosion inhibitors used in the sulfuric acid medium of ditTerentconcentrations has already been given by Frenier and Growcock88 in their review on corrosion inhibitors for cleaning(pickling) acid media in the previous corrosion inhibitor review symposium. Some additional data is provided here. Sulfuricacid corrosion also can be handled by using azoles and hctemcyclic compounds containing both nitrogen and sultir atoms,a topic that is reviewed in this symposium by Ajmal, et al.89

Inhibition aspects of various engineering alloys, particularly stainless steels, in acid media are covered in the paperby Trabanelli, et al.,w which can also be referred to for very useful information on CIS and their performance in acid media;though the acids covered in that paper are differen4 usefid data on CIS for some other important acid media, such as HC1,HF, sulphamic acid, etc., can be obtained.

Aeration or addition of suitable amounts of an oxidizing agent stabilizes the Cr-nch protective oxide film formedon stainless steelsgl and thus can help in inhibiting corrosion to some extent. Oxidizing agents such as ferric, cupric, andstannic ions which can be easily reduced inhibit corrosion.m Nitric acid at concentration levels as low as 1.5% is reported toinhibit the corrosion of stainless steels over a wide range of sulfhric acid concentration at ambient and elevatedtemperatures.65 Anodic protection has been used as an effective measure to inhibit corrosion in sulfuric acid plants.932% boron trifluoride has been recommended as an inhibitor for 80A SA.94 For 60-70% concentration arsenic isrecommended.gs Silicon added to some special steels and stainless steels meant for SA sewice leads to the formation of atenacious passive film containing silico~ during the initial stages of corrosio% and thereby inhibit firther corrosion.Resistance to localized corrosion, pitting and erosion corrosion is also strengthened. In cases where severe corrosion keepson occurring, zirconium alloys that have superior corrosion resistance for SA can be used.

Stainless steels are used for handling dilute SA along with CIS such as aromatic tines.% For SA solutionscontaining NaCl, some aniline derivatives, benzaldehyde and pyrillium salts have been recommended as CIS.762-mercaptobenzimidazole, l-mercaptobenzothiazole, and thiourea derivatives have been reported to give promising resultsin SA solutions containing NaC1.97 Stainless steels are susceptible to SCC in SA solutions containing chloride ions.

O-tolyl thiourea is another CI which can inhibit corrosion of mild steel in SA.m This compound is more effectiveat 80C than at 40C. Arnines, aldehydes, etc., did not show any such temperature dependence. The finding with thethiourea CI was attributed to slower diffusion of fairly large inhibitor molecules and to higher activation energy of theadsorption process,% and the elevated temperature would help in activating diffusion and adsorption.

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Nitric acid (NA): Nitric acid is used as a feedstoekin the manufactureof fertilizers such as nitrophosphatesW)and other compound fertilizers containing also potash (NPK) by the phospho-nitric process.wlw Fertilizer plants produceNA by the oxidation of ammonia which has a concentration of 60Y0. This acid is used primarily to produce ammoniumnitrate and for the digestion of phosphate reek to produce PA. 1m The NA may further be concentrated to produce fumingacid. A good reviewon the materialsof constructionfor NA seMee is available.1Nitric acid up to gs~. strength is storedand shipped in Type 304 stainless steel. Aluminum alloys are also used for the catalytic decomposition of ammonia and inthe processing of ammonium nitrate.02

Nitric acid is a strong oxidizing agent and forms an oxide film on austenitic stainless steels or on aluminum alloys,which inhibits corrosion. The passivated stainless alloys used in the NA service sometimes show selective intergranularcorrosion, especially in strong NA. Low carbon levels, and additions of silicon are reported to help in the inhibition of suchlocalized corrosion.103 Also, sensitization of stainless steels needs to be avoided through proper heat treatments. Exposure toNA could lead to hydrogen embnttlement in high strength steels due to evolution of hydrogen. Nitric acid is highlycorrosive to aluminum in the 20-30/0 concentration range. ]w Hydrofluoric acid is used with Al-alloys as an inhibitor instorage and shipping of fuming NA at temperaturesup to 70C.8Unalloyedtitanium showsexcellentperformanceover thecompleterange of concentrationat temperaturesup to 80C;yet, this dependson the purity of the acid. But, in fuming NA,stress corrosion cracking is possible. Small amounts of impurities, such as Fe3+, Si4+, etc., can effectively inhibit corrosionof Ti and its alloys by NA. Also, titanium corrosion in NA can be inhibited by its own corrosion product Ti4+. Addition of2% water to the fuming NA has been recommended as an inhibitor for titanium.los

Some of the common inhibitors used in the nitric acid arena are discussed in the review by Frenier andGrowcock,88 which can be consulted. They are also listed in Table 3.

Other installations:

Some of the other major installations that require corrosion protection and application of CIS include the boilersand cooling water systems, supporting structures and Betileld plant for C02 absorption.

Boiler and cooling water systems: These use the conventional procedures for corrosion control. The materials ofconstruction in boilers are mostly carbon steels or low alloy steels. These sutTer pitting, SCC, caustic embrittlement, andsevere erosion corrosion problems. General corrosion is not a serious problem in boilers. Failure often occurs due tolocalized corrosion and erosion. Tipnis et al.sb have reported failure of reformed-gas boiler tubes leading to leakages. Thefailure has been attributed to pitting from the water-side surface, underneath deposits. Phosphate salts were used for thecontrol of water chemistry. The authors have recommended the use of volatile arnines, although these were not tried in theinstallation involved.

In cooling water systems, which include heat exchanger units, large quantities of CIS are used. The corrosion andits inhibition here is not spee~lc to fertilizer production and handling. The corrosion problems are influenced as usual bythe scales, sludges, deposits, eontarninations, bacterial growth etc., all of which interfere with the corrosion inhibitionprocess. Commonly used water treatments are applied here. Chromate treatment was being used about twenty years ago.Rajagopalan et al.lw discuss a treatment with chromate, followed by ferrous sulfate and reeycling, to prevent discharge ofCr& ions into the river in a fertilizer plant in Mangalore, India. A non-chromate-based water treatment for cooling water inan ammonia and urea manufacturing plant was recently described by Dubey and Mittal. ]07

COZ absorption: COZremoval from the process gas in the Bentleld plant involves operations of absorber units,which require corrosion protection. This is achieved by the use of passivators such as pentavalent vanadium-containing salesodium metavanadate, at concentration levels of about O.S/O.] lW Effectiveness of V* ions of the inhibitor is tiected bychloride ion concentration in the stream. The oxidized form of vanadium needs to be maintained for preserving itsefllciency. The eflicieney would be lost when the V= concentration falls below 2000 ppm.

Supporting structures: Metallic and concrete structures which support machinery such as pumps, compressors,pressure vessels, storage tanks, pipelines, heat exchangers, etc., are also susceptible to corrosion due to spillages ofcorrodents, stagmtion of products, etc., as already pointed out. Failure has been reported at rivets, welds, etc., criticalplaces. The structures are protected using special paint matings and linings, which use inhibitors in the primer coats.

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Fitzsimmons and Henry* 10have discussed the maintenance of concrete materials in support structures using urethane,epoxy, acrylic, asphalt etc., in suitable paint systems in an ammonium nitrate plant environment.

Verma and Chakrabortyl *1 reported that Ca ferrite pigment in the prime coat offered good protection in a fertilizerplant environment near the urea, SA, N~ and NPK ( compound fertilizer containing N,P, and K) production facilities, butblistering and rust formation occurred in the area of ammonium nitrate production. Zinc ferrite pigment was found to begood for the environment in the ammonium nitrate, SA, and NA plants, but failed in the urea and NPK plant environments.Ca-fernte was superior to Zn-ferrite and both were better than Zn-chromate and red lead PbsO~.

CONCLUSION

A great deal of information is available on the applicability of CIS in the fertilizer industry. The choice ofinhibitors is obviously tied to the particular corrosion problem involved. The latter vary from general corrosion, to veryserious stress corrosion cracking and erosion corrosion, etc. Contaminants are natural and cannot be always eliminated.The choice of a suitable strategy to combat the corrosion problems appears to be influenced by the considerations ofeconomics, safety of the installations and the understanding of the mechanisms of the corrosion processes. Selection ofsuitable long-range corrosion resistant materials, though it would lead to increased cost, seems to be the most favored onefor handling the most corrosive chemicals, such as strong acids. Even such corrosion resistant materials are sometimes usedin conjunction with corrosion protective measures to prevent unforeseen problems. Application of CIS in such instances is asimple and a practical choice. Though the use of CIS is in vogue, the mechanisms of action of the CIS, as well as theirlimitations, are less well understood in the contexl of the fertilizer chemicals, particularly the compound fertilizers.

Problems often persist and the solutions offered are found to be inadequate. This is attributed to local variations ofprocess parameters, the chemistry of the corrodents and the strength and depletion of inhibitors, ingress of contaminantsand presence of impurities, trace additions of other constituents, residual and seMce stresses, material sensitivities, etc.Failures in the ammonia storage tanks, or leakages in the SA handling systems, are some of the examples.

The detection and monitoring of corrosion damage appears to lx well handled through utilization of various non-destructive test techniques. Information on the functional features of CIS during application is not readily monitorable. Incritical applications, such as in pressure vessels, suitable monitoring techniques to assure the effkacy of functioning CIS areneeded. Data on threshold stress intensity levels for initiating various localized cracking processes in the presence of variousinhibitors and at their various concentrations would be very usefid. Data on the synergistic effects among various inhibitorsin the fertilizer plant chemicals, with the effects of velocity of corrosive fluids and the various impurities involvedsuperimposed, would also be equally usefid. Many of the formulations are marketed under trade names and complete dataon their constituents and compositions are not available. This forms another handicap to the consumer and he is asked torely upon the experience of the producer. Oftentimes the middlema~ who handles the distribution of the CIS, is notknowledgeable enough and the optimum choice is not made. Though this is a universal trend nevertheless, vast gains havebeen made in the applications of efficient inhibitor formulations in the fertilizer indust~ in the production and handling offertilizers.

REFERENCES

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23219

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54. R.F. Miller and M.W. Blaschke, US Pat. US 4,781,748; Nov. 1988.55. B.A. Gru, et al., Russ. Pat. SU 1,781,193; Dec. 1992.56. C. Rajagopal, et al., Bull. Electrochem,, 6,1 (1990): p. 23.57. N.C. Subramanyan, B.S. Seshadri and S.M. Mayanna, Corr. Sci., 34,4 (1993): p. 563.58. L.E. Ott, US Pat. US 4,256,691; March 1987.59. Corrosion Inhibitors and Temporary Protection Media, Scientiilc papers No.33, Conf. No. 17, S. Poreba, J. Kubicki,S. Kuczkowska and L. Wedzicha, eds. (Wrozaw, Poland: Politehniki Wroclauskiej, 1987) (CA: 108-B 9941 It)60. B. Neveu, Valorization of phosphogypsum, in New Developments in Phosphate Fertilizer Technolo@ (New York:Elsevier, 1977), p. 31.61. S.B. Parikh, Processing of standard rock, other rocks and rock blends in the phosphoric acid plant, ibid, p. 289.62. A. Alon, J. Yahalon and M. Schortj Corrosion, 31 (1975): p. 315.63. W.P. Banks and E.C. French, Materials Protection, 6 (1967): p. 48.64. RE. Jenkins, J.H. Karrh and C.H. McCullough, U.S. Patent US 1,809,041; June 1931.65. F. Flade, The passivity of electrodes, Z. Phys. Chem., 76 (1911): p. 513.66. B. Linder, Anodic protection of stainless steels in HsPOa containing halide ions, Ind. Corr., 5,9 (1987): p. 12.67. T.P. Hignet~ E.C. Doll, O.H. Livingston and B. Reistrick, Utilization of difllcult phosphate ores, in NewDevelopments in Phosphate Fertilizer Technology (New York: Elsevier, 1977), p. 273.68. A.C. Hart, Brit. Corr. J., 8,3 (1973): p. 66.69. Pierre Becker, Phosphate and Phosphoric Acid (New York: Marcel Dekker, 1983), p. 497.70. A. Lizlovs, Corrosion 29 (1967): p. 389.71. A.C. Hart, Brit. Corr. J., 6 (1971): p. 207.72. A.A. E1-Hosary, M.M. Badran, R.M. Saleh and H.A. E1-Dahan, in Proc. 10th Intemat. Congr.Met. Corr., Madras, India, Vol. III (New Delhi, India: Oxford& IBH, 1987), P.2787.73. A. Hochmann and A. Desestret, La Metallurgic, 99,2 (1967): p. 111.74. I.P. Anoshchenko, V.P. Houston, E.V. Pnzey and A.V. Andresov, in Proc. 5th Euro. Syrup. on Corr. Inhibitors, Ferrara(Fcrrara, Italy: Univ. Fcrrara, 1980), p. 73.75. A. Bellaouchou, A. Grcnbour and A. Benbachir, Corrosion, 49,8 (1993): p. 656; J.P. Audouard, D. Catelin andP. Soulignac, in Corrosion Prevention in the Process Industries, RN. Parkins, ed. (Houston, TX: NACE, 1990), p. 71.76. Corr. Inhib. Develop. since 1980, M.J. Collie, ed. (New Jersey: Noyes Data Corp., 1983), p. 52.77. E. Pelitti, Corrosion and materials of construction in phosphoric acid, ed. A.V. Slack, Vol. 1, Ch. 10,(New York: Marcel Dckker, 1988).78. A. Guenbour, J. Fauchen, A. Ben Bachir, F. Dabosi and N. Bui, Brit. Corr. J., 23 (1988): p. 234.79. K.M. Verma, H. Ghosh, K.C. Patnaik and RV. Singh, ibid, 17,2 (1982): p. 1071.80. J.C. Charbonnier, Met. Corr. Ind., 398 (1976): p. 201.81. N. Bui, A. Irhzo, F. Dabosi and L. Maire, Corrosio% 29 (1983): p. 491.82 M. Autti, M. Loikkanen and P. Suppanen, in NCW Developments in Phosphate Fertilizer Technology, L. J. Carpenter,ed. (NCWYork: Elsevier, 1977), p. 234.83. J.R. Home and L.S. Houille, ibid, p. 38.84. J.C. Nickerson, ibid, p. 88.85. R.J. Borges, ibid, p. 81.86. A.B. Tipnis, D. Deshmukh and N.C. Bandopadhyay, Ammonia Plant Safety, AIChE, 34 (1994): p. 271.87. L. Friedman, Adv. Phosphate Fertilizer Tech. AIChE, 89 (1993): p. 6.88. W. W. Frenier and F.B. Growcock, in Reviews on Corrosion Inhibitor Science and Technology,A. Raman and P. Labine, eds. (Housto% TX:NACE International, 1993), 11-20-1.89. M.J. Ajmal, et al., Corrosion-96 paper#217 (Houston, TX: NACE, 1996).90. G. Trabanelli, F. Zucchi, A. Frignani, G. Brunoro, C. Monticello and M. Zucchini, in Corrosion Prevention in theProcess Industries, R.N. Parkins, ed. (Houston, TX: NACE, 1990), p. 145.91. V.P. Grigoryev, V.V. Ekilik, G.N. Ekilik and J.B. Kudryashova, Proc. 4th Euro. Symp. Corr. Inhibitors, Ann. Univ.Ferrara, NS Ser. V, 6 (1971), p. 369.92. F. Zucchi, G. Trabanelli, A. Frignani and M. Zucchini, Corr. Sci., 18 (1978): p. 877.93. E.H. Ferking and R.E. BoillaL Adv. in Phosphate Fertilizer Tech., AIChE Symp. Ser., 89 (1993), p. 194. R.A. Bakeer, US Patent US 2, 572, 301; Oct. 1951.95. Wechter, Tresedcr and Wcber, Corrosion 3 (1947): p. 406.96. M.A. McDermatt U.S. Patent US 1,927,842; Sept. 1933.97. G. Trabanelli, A. Frignani, M. Zucchini and F. Zucchi, in Proc. 9th Intemat. Congr. Metal. Corr.(Ottawa, Canada: Nat. Res. Council, 1984), p. 230.98. T.P. Hoar and R.D. Halliday, J. Appl. Chem.,(1953): p. 502.

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99. F. Codina and S.A. Cros, in New Developments in Fertilizer Technology (New York: Elsevier, 1977), p. 234.100. F.G. Mcmbrellera and J.L, Torel, ibid, p. 254.101. R. Crooks, Materials for construction for nitric acid in process industries, in Corrosion- The Theory and Practice,B.J. Miniz and W.I. Pollok, eds. (Houston, TX: NACE, 1986).102, C. Chakraborty, M.M. Singh and C.V. Agarwal, in Proe. Corrosion Inhibition Conf., Dallas, 1983(Houston, TX: NACE, 1988).103, C.P. Dillon, Mater. Perform., 31,7 (1992): p. 51.104, E.H. Cooks, Jr., RC. Horst and W. W. Binger, Corrosion, 17,1 (1961): p. 25t.105, J.B. Rittenhouse and C.A. Papp, Corrosion, 14 (1958): p. 283.106. T.V. Rajagopalan, et al., Proe. Fertilizer AssIx. India, XXI, 1976.107. M.N. Dubey and P. Mittal, Fertilizer News, 35,7 (1990): p. 15.108. J.N. Iyengar, D.E. Yoga and P.S. Yoga, Ammonia Plant Safety, AIChE, 34 (1994): p. 177.109. E. Lumarska and A. Jzyprowski, Brit. Corr. J., 29,3 (1994): p. 776.110. T. Fitzsimmons and N. Heruy, in Corrosion Prevention in the Process Industries, R.N. Parkins, ed. (Houston, TX:NACE, 1990), p. 313.111. K.M. Vcrma and B.R. Chakraborty, Anti Corr. Methods Mater., 34,6 (1987): p. 4.

Table 1Some Important Types of Common Fertilizers.

Types of Fertilizers Common Name/Composition

Mixtures NP, w P~ NPK (indicating Nitrogen (N),Phosphorus (P), and Potassium (K) in the mix)

Chemical Nitrogen Materials Ammonia, anhydrousAmmonia, aqueousAmmonium nitrate; ammonium sulfate;ammonium chlorideCalcium ammonium nitratenitrogen solutionssodium nitrateUrea

Phosphate Materials Ammonium phosphate; di-ammonium phosphatephosphoric acid (PA)nitro phosphate; super phosphate

Potassium Potash

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Table 2Raw Materials Required to Produce 1 Ton of IYPK 13-12-12 grade Fertilizerl

Raw Material Amount needed(in: lbs/ ton of fertilizer made)

Nitric acid 612Anhydrous ammonia 166Phosphate rock 660Muriate of potash 410C02 130Stabilizer chemicals 70

Table 3Summa~ of Important CIS Used in the Fertilizer Plant in Various Locations

Plant Location CIS COInInOtiy USed

Ammonia plant/storage tanks,pipelines, etc.

Ammonium nitratrdammonium nitrattiurea mix.

Phosphoric acid

Sulfuric acid

Nitric acid

C02 absorption units

Paint systems on structures

O.2%Owater, hydrazine, vegetable oils, ammonium chloride

thiourea, ammoniumthiocyanate,2-mercaptobenzothiazoleorthophosphates/phosphateesters; alkylammoniumphosphates;alkylbenzenesulfonates,polyvalentaliphaticacids, alkoxylatedmmpounds, fatty acid sulfate,alkylsulfonylglyeines,NaNOz-earbonateJZnS04mixtures;amine oxide,etc.

selenols,thiocyanates, thiols, thiocarbonyls, aldehydes,arnines, inorganic passivators, KI, NI-LI, NaI, etc.,HN03, fatty acid+ ICImixes, Na-bichromate, etc.

azoles, thiazoles,heteroeycliccompoundscontaining Nand S, Nitric acid, thiourea derivatives,aniline derivatives,benzaldehyde,etc.

mercaptons, thiols, sulfldes, pyridine thiocyanatederivatives,arnk+aldehyde+acctylinic alcohols,thiourea derivatives,etc.

0.5% V5+ from sodium metavanadate

Ca-, and Zn- ferrite pigments

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