Cor­ro­sion & coat­ings

DEMM Engineering & Manufacturing - - CONTENTS -

Through­out the Asia Pa­cific re­gion there are tens of thou­sands of bridges and re­lated road and rail in­fra­struc­ture. The va­ri­ety of de­signs and con­struc­tion ma­te­rial used to build these as­sets present a wide range of chal­lenges to the peo­ple charged with man­ag­ing and main­tain­ing them. Degra­da­tion of bridges is caused by many dif­fer­ent fac­tors in­clud­ing cor­ro­sion and other stresses from both the en­vi­ron­ment and heavy ve­hi­cles pass­ing over them.

In Aus­tralia, the yearly cost of as­set main­te­nance is es­ti­mated to be ap­prox­i­mately AUD32 bil­lion. Avoid­able cor­ro­sion dam­age ac­counts for AUD8 bil­lion of this and con­tin­ues to have a ma­jor eco­nomic im­pact on in­dus­try and the wider com­mu­nity. The pro­por­tional costs and im­pact of cor­ro­sion are sim­i­lar for most coun­tries in the Asia Pa­cific re­gion.

Cor­ro­sion will af­fect all types of met­als to vary­ing de­grees of sever­ity and speed. Un­less com­pre­hen­sive man­age­ment plans are de­vel­oped and im­ple­mented, steel and other met­als will ‘rust’ and re­in­forced con­crete will spall and crack. Cor­ro­sion can be pre­vented or min­imised by ei­ther ‘iso­lat­ing’ the ma­te­rial from its en­vi­ron­ment with some sort of coat­ing or im­ple­ment­ing an ac­tive in­ter­ven­tion sys­tem such as ca­thodic pro­tec­tion.

The en­vi­ron­ment and pre­vail­ing cli­matic con­di­tions also con­trib­ute to the degra­da­tion of bridges. The largest cities in the re­gion are ei­ther in coastal or trop­i­cal zones, with some even ex­posed to the com­bi­na­tion of both. Bridges in Dar­win and Bris­bane, along with many other cities through­out the re­gion, can be im­pacted by ex­treme wind speeds of trop­i­cal storms in ad­di­tion to the high lev­els of air­borne salt found in coastal lo­ca­tions.

Harsh en­vi­ron­ments – es­pe­cially with high chem­i­cal lev­els or ex­treme tem­per­a­tures – can ac­cel­er­ate rates of cor­ro­sion.

Bridges also carry mas­sive loads from mov­ing ve­hi­cles which im­pose vi­bra­tional and other stresses onto struc­tures. Ap­prox­i­mately 200,000 cars and trucks cross Melbourne’s West­gate Bridge each day, mak­ing it one of the coun­try’s busiest road cor­ri­dors. Sydney’s Har­bour Bridge car­ries 160,000 ve­hi­cles each day be­tween North Sydney and the CBD as well as 204 trains. The Auck­land Har­bour Bridge car­ries a sim­i­lar vol­ume of road traf­fic, although it is es­ti­mated that half the peo­ple cross­ing the bridge in the morn­ing peak hour are on buses.

The own­ers and man­agers of these as­sets must en­sure that bridges are safe, while main­tain­ing ac­cept­able lev­els of ser­vice for the du­ra­tion of the ex­pected life of the as­set. If ap­pro­pri­ate as­set man­age­ment strate­gies are im­ple­mented, it is pos­si­ble to re­store an as­set to near its orig­i­nal con­di­tion and main­tain its func­tion­al­ity for the re­main­ing ser­vice life and, pos­si­bly, even be­yond.

Work­ing with in­dus­try and academia to re­search all as­pects of cor­ro­sion, the Aus­tralasian Cor­ro­sion As­so­ci­a­tion Inc. (ACA) pro­vides an ex­ten­sive knowl­edge base that sup­ports best prac­tice in cor­ro­sion man­age­ment, thereby en­sur­ing all im­pacts of cor­ro­sion are re­spon­si­bly man­aged, the en­vi­ron­ment is pro­tected, pub­lic safety en­hanced and economies im­proved.

Recog­ni­tion of the need to ef­fec­tively main­tain road and rail in­fra­struc­ture is in­creas­ing. An illustration of this is the an­nounce­ment by the Aus­tralian Fed­eral govern­ment of fur­ther fund­ing of its na­tional Bridges Re­newal Pro­gram. Dar­ren Ch­ester, for­mer Fed­eral Min­is­ter for In­fra­struc­ture and Trans­port, said that the Aus­tralian Govern­ment’s fund­ing would see an ad­di­tional 186 projects added to the re­place­ment or up­grade work be­ing car­ried out on 201 bridges al­ready. The new fund­ing is in ad­di­tion to the AUD216 mil­lion al­ready com­mit­ted un­der the first two rounds of the pro­gram.

An­other was the ini­ti­a­tion by Raed El Sar­raf, Cor­ro­sion and As­set In­tegrity Con­sul­tant with WSP Opus in New Zealand, of a Big Bridges Work­shop in 2017 that was held in Sydney and at­tended by rep­re­sen­ta­tives of the stake­hold­ers in the larger, iconic bridges in the re­gion, in­clud­ing the Sydney Har­bour Bridge, Auck­land Har­bour Bridge, Bris­bane’s Story Bridge and Melbourne’s West­gate Bridge.

The two most com­mon causes of con­crete cor­ro­sion are car­bon­a­tion and chlo­ride or ‘salt at­tack’. The al­ka­line (high pH) con­di­tions in con­crete forms a pas­sive film on the sur­face of the steel re­in­forc­ing bars, thus pre­vent­ing or min­imis­ing cor­ro­sion. Re­duc­tion of the pH caused by “car­bon­a­tion” or ingress of chlo­ride (salt) causes the pas­sive film to de­grade, al­low­ing the re­in­force­ment to cor­rode in the pres­ence of oxy­gen and mois­ture. Leach­ing of the al­ka­lin­ity from con­crete also low­ers pH to cause cor­ro­sion of steel re­in­force­ment. Stray elec­tri­cal cur­rents, most com­monly from elec­tri­fied trac­tion sys­tems, can also break­down the pas­sive film and cause cor­ro­sion of steel re­in­forced con­crete and pre­stressed con­crete el­e­ments.

As re­in­forc­ing bars rust, the vol­ume of the rust prod­ucts can in­crease up to six times that of the orig­i­nal steel, thus in­creas­ing pres­sure on the sur­round­ing ma­te­rial which slowly cracks the con­crete. The most ex­posed el­e­ments usu­ally de­te­ri­o­rate first and it may take five to 15 years for the ef­fects of re­in­forc­ing steel cor­ro­sion to be­come vis­i­bly no­tice­able. Cracks even­tu­ally ap­pear on the sur­face and con­crete starts to flake off or spall.

War­ren Green, Direc­tor and Cor­ro­sion En­gi­neer at en­gi­neer­ing con­sul­tancy firm, Vinsi Part­ners, stated that not all cor­ro­sion of re­in­force­ment leads to vis­i­ble rust stain­ing, crack­ing, de­lam­i­na­tion or spalling of cover con­crete. Sig­nif­i­cant sec­tion loss can also oc­cur where there is lo­calised pit­ting or lo­calised cor­ro­sion at cracks and sur­face de­fects. Ul­ti­mately, struc­tural fail­ure may oc­cur with­out any vis­i­ble con­se­quences of cor­ro­sion on the sur­face of the con­crete. Pits usu­ally start out quite nar­row, but with time co­a­lesce to form larger ones and re­sult in sec­tion loss over a greater (an­odic) area.

Green stated that var­i­ous re­pair and pro­tec­tion tech­nolo­gies and ap­proaches are pos­si­ble dur­ing the life­time of a re­in­forced con­crete struc­ture, de­pend­ing on the type of cor­ro­sion mech­a­nism. Re­me­dial op­tions avail­able that can slow the rate of re­in­force­ment cor­ro­sion in­clude coat­ings, pen­e­trants, waterproofing, cor­ro­sion in­hibitors, elec­tro­chem­i­cal (gal­vanic an­odes) and elec­tro­chem­i­cal (hy­brid treat­ment).

There are also re­me­dial op­tions to stop cor­ro­sion of re­in­force­ment. These in­clude ca­thodic pro­tec­tion, elec­tro­chem­i­cal chlo­ride ex­trac­tion and elec­tro­chem­i­cal re-al­ka­li­sa­tion.

In ad­di­tion to the range of re­pair and pro­tec­tion ap­proaches, the lat­est con­crete struc­tures in­cor­po­rate new ma­te­ri­als and pro­duc­tion meth­ods which im­prove longevity and per­for­mance. As a re­sult of the re­search into con­crete ad­di­tives, con­struc­tion com­pa­nies and en­gi­neer­ing con­sul­tan­cies have ac­cess to all the lat­est tech­nolo­gies that yield a suite of proac­tive and re­ac­tive pro­cesses and pro­ce­dures to max­imise the dura­bil­ity of re­in­forced and pre-stressed con­crete.

The phys­i­cal as­pects of ap­ply­ing a coat­ing or re­pair­ing a sec­tion of steel or con­crete present their own chal­lenges for own­ers and op­er­a­tors of bridges. The tow­ers and stays of sus­pen­sion-type bridges of­ten re­quire staff to have ad­vanced ab­seil­ing skills so they can ac­cess them. Metal struc­tures usu­ally need spe­cialised equip­ment and scaf­fold­ing to al­low work­ers to safely per­form main­te­nance work.

New Zealand has ap­prox­i­mately 2300 bridges of vary­ing size as­so­ci­ated with the coun­try’s high­ways. A large pro­por­tion of the bridges are con­crete decks on steel frames and sup­ports or pre-stressed con­crete struc­tures, in ad­di­tion to bridges made of con­ven­tional re­in­forced con­crete and tim­ber. Ac­cord­ing to Wil­lie Man­deno, Prin­ci­pal Ma­te­ri­als and Cor­ro­sion En­gi­neer with WSP Opus, the main­te­nance and mon­i­tor­ing of these struc­tures con­tin­u­ally adapts to chang­ing con­di­tions and tech­nolo­gies.

The iconic Auck­land Har­bour Bridge is a steel truss and box girder de­sign. For many years, the main­te­nance of this bridge in­volved a con­tin­u­ing pro­gram of paint­ing, where ap­pli­ca­tors started at one end and when they got to the other end, went back to the begin­ning again. Ac­cord­ing to Man­deno, this has changed. “Old oil-based paints be­came very brit­tle and could crack, then de­lam­i­nate,” he said. “In the late 1990s they changed to a mois­ture-cured ure­thane which gives ap­prox­i­mately a 20-year life­span be­fore the bridge needs to be re­painted.”

While the time be­tween re­coat­ing is now much longer, it is still nec­es­sary to con­tin­u­ally mon­i­tor the old coat­ings to en­sure ad­he­sion is main­tained. “When re­coat­ing, the ideal is to just re­place the top coat,” Man­deno said, “but we usu­ally have to do some main­te­nance work first, such as clean­ing and reprim­ing of edges and around rivet heads.”

Early sol­vent-based paints used to con­tain chro­mates and lead, along with a range of other haz­ardous chem­i­cals. “We have had to bal­ance pro­tect­ing the en­vi­ron­ment with the re­duced per­for­mance of wa­ter-based coat­ings,” Man­deno said. “One sol­vent-free long-life coat­ing that we now rec­om­mend for use in coastal ar­eas is ther­mal sprayed zinc. One lim­i­ta­tion of this ma­te­rial, and the al­ter­na­tive high-build in­or­ganic zinc sil­i­cate coat­ings, is that it is that they are only avail­able in shades of grey.”

Many roads through­out the re­gion are be­ing up­graded to al­low for longer and heav­ier trucks. All road au­thor­i­ties face sim­i­lar chal­lenges when man­ag­ing the risks of age­ing in­fra­struc­ture de­signed to a much lower stan­dard, whilst still pro­vid­ing ac­cess for mod­ern heavy ve­hi­cles.

Short span struc­tures like cul­verts are only ex­posed to one axle group at any one time whereas longer span struc­tures built dur­ing the past cen­tury are now re­quired to carry sub­stan­tially more load than they were orig­i­nally de­signed for.

In New Zealand, Man­deno stated that many of the older tim­ber rail bridges near­ing the end of their use­ful life are be­ing re­placed by ‘weath­er­ing steel’ girder bridges which should pro­vide a longer op­er­a­tional life­span.

Of­fi­cially known as “struc­tural steel with im­proved at­mo­spheric cor­ro­sion re­sis­tance,” weath­er­ing steel is a high strength, low al­loy steel that, in suit­able en­vi­ron­ments – those not ex­posed to high lev­els of salin­ity and pol­lu­tants – may be left un­painted al­low­ing a pro­tec­tive rust “patina” to form and min­imise fur­ther cor­ro­sion. Al­loy com­po­nents such as cop­per, chromium, sil­i­con and phos­pho­rus form less than two per cent of the steel but it re­tains ap­pro­pri­ate strength, duc­til­ity, tough­ness and weld­abil­ity so that it can be used for bridge con­struc­tion.

All struc­tural steel rusts at a rate de­ter­mined by the amount of mois­ture and oxy­gen to which the metal­lic iron is ex­posed. As this process con­tin­ues, the ox­ide (rust) layer be­comes a bar­rier re­strict­ing fur­ther ingress of mois­ture and oxy­gen to the metal, and the rate of cor­ro­sion slows down.

The rust layer that forms on most con­ven­tional car­bon­man­ganese struc­tural steels is rel­a­tively por­ous and flakes off the sur­face al­low­ing a fresh cor­ro­sion cy­cle to oc­cur. How­ever, due to the al­loy­ing el­e­ments in weath­er­ing steel, a sta­ble rust layer is pro­duced that ad­heres to the base metal and is much less por­ous. This layer de­vel­ops un­der con­di­tions of al­ter­nate wet­ting and dry­ing to pro­duce a pro­tec­tive bar­rier which im­pedes fur­ther ac­cess of oxy­gen and mois­ture. It is pos­si­ble that if the rust layer re­mains suf­fi­ciently im­per­vi­ous and tightly ad­her­ing, the cor­ro­sion rate may re­duce to an ex­tremely low one.

It can be rel­a­tively sim­ple to cal­cu­late loads and stresses on bridges when weights are dis­trib­uted evenly across the struc­ture, but road au­thor­i­ties also have to deal with heavy and over-di­men­sion loads. Move­ment of such ve­hi­cles re­quires spe­cial plan­ning as there are some roads and bridges that are phys­i­cally un­able to sup­port mas­sive weight con­cen­trated into a small area.

Mod­ern tech­nol­ogy can as­sist in man­ag­ing some struc­tures sen­si­tive to vi­bra­tion from heavy ve­hi­cles. Elec­tronic sen­sors can be set up to mon­i­tor vi­bra­tions and other stresses on struc­tures so that a large num­ber of data points are logged that can be down­loaded for analysis. Sen­sors can also be con­nected to re­mote cam­eras that are trig­gered when­ever a thresh­old vi­bra­tion level is ex­ceeded to iden­tify which ve­hi­cles are pro­duc­ing these ef­fects.

It is strongly rec­om­mended that a dura­bil­ity plan be de­vel­oped which then be­comes a crit­i­cal tool in sup­port­ing an over­ar­ch­ing as­set man­age­ment strat­egy. The plan should clearly out­line likely cor­ro­sion-re­lated risks and agreed mit­i­ga­tion ap­proaches as early as pos­si­ble in an as­set’s life­cy­cle, ide­ally dur­ing the plan­ning and de­sign stage.

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