VTCCRD Austin Metro Corrosion testing

he's living this new five-door model of the austin metro is fresh off the production line but just 12 grueling weeks from now and it'll be on the scrap heap [Applause] having deliberately chipped the paintwork this car wash has something special to offer salt water these are just some of the stages of the accelerated corrosion program used by austin rover over the next 12 weeks this car will age through the equivalent of six canadian winters the objective is to pinpoint potential corrosion problems and every prototype is tested as well as established models mud and mechanical vibration add to the car's problems but there's worse to come humidity and high temperature increase the rate of corrosion models under test are kept overnight in this special chamber it's rather like a sauna and here's the result one not so careful owner only three thousand miles but bodywork in need of some attention price well for austin rover the price of a new car was money well spent if it leads to better cars that corrode less more cars end up here because of corrosion than for any other single reason it's not that they're rusting more quickly but rather that their mechanical parts last longer than they used to corrosion isn't attractive but its real threat is that it weakens the structure the results are potentially fatal [Applause] so what are the manufacturers doing about it what can you do about it let's start right at the beginning [Applause] [Music] at austin rovers longbridge plant the morning shift is underway three freight trains arrive here every day carrying components pressed from sheep steel that in just a few hours time will look rather more familiar these components have traveled from swindon in a controlled environment and are loaded and unloaded indoors any water at this stage would be very bad news indeed the sheet steel is only seven tenths of a millimeter thick and apart from a light covering of oil it's unprotected in fact the whole car body is made from steel of this thickness but still in this state will rust this unprotected test piece was attached to a car undergoing the accelerated corrosion test twelve weeks later it's extensively corroded and very seriously weakened but preventing this happening to your car is not just about protecting the steel directly painting it for instance this is austin rovers computer-aided design department at swindon as we follow the production line at longbridge we'll see just how important it is to get the design right it's not too difficult to protect a good design but it's impossible to correct a bad one later in production so what does the designer need to be aware of what's the underlying chemistry if we place a piece of steel in an atmosphere of oxygen then a film of oxide is slowly produced but this looks very different from rust if we leave the steel in water then again virtually no rust is formed but the water had been degassed so there was no oxygen present but if both water and oxygen are present then rust forms very quickly to see exactly what is happening we use a special feroxial indicator this time we just need a single drop of water the blue in the center means ferrous ions are being formed the indicator turns them this bright color around the perimeter a pink coloration the indicator is showing us the presence of hydroxide ions within this single drop a short-circuited electrochemical cell has been formed but how exactly well in the center this oxidation reaction is taking place the electrons produced are conducted by the metal to the reduction reaction taking place at the edge hydroxide ions are only produced at the edge of the drop because it's here that oxygen is most available the circuit is completed by the ions moving between the electrodes within the solution so to break this electrochemical circuit we must keep moisture away from bare steel if we don't then this is the result the sills of a car are notoriously vulnerable partly because they're hollow and can thus trap moisture and also because they're in an exposed position underneath the car here's what can happen despite strenuous efforts with what looks like papier-mache these sills are completely rotten there's a danger that sills will accumulate water inside so special drainage holes are provided which by sloping backwards help prevent water getting back in another idea tried in some models is that of forced ventilation while the car is moving air passes from a grill in the bonnet down the a post and then along the sill this helps keep the sill dry and therefore rust free but there are other places where corrosion can attack wherever there's a closed section the potential exists for moisture to be trapped and for corrosion to start inside that box so how can the designer avoid a rust trap on the metro line at long bridge this box section is about to be welded to the main floor assembly but it's not really a box at all the sides have been completely cut away like this though it would be an instant trap for water and mud but the designers have another trick up their sleeves by placing this section inside the car it's protected it's not in the way because it runs across the car underneath the front seats but what if you can't hide things inside the car wheel arches are vulnerable to corrosion because of all the water and mud thrown up by the wheels but where exactly does corrosion occur just underneath this front fender is a seam water thrown up by the wheel has got into that seam and corrosion products are actually forcing the seam apart but it's mud caked onto the car that keeps the wheel arch damp and speeds up corrosion it's mud thrown up around the back of the headlight fixture for instance that has caused this design is all important in avoiding problems on some models to deflect mud water and chippings thrown up off the road austin rover are fitting wheel arch liners seams are always especially vulnerable to corrosion it's difficult to exclude water especially from forward-facing seams on this door for instance the frame around the window is a separate component from the door itself and water has leaked in there all the various joints yeah that's right it's a big contrast to the one piece pressing where we've got very clean condition at the front there a one-piece pressing with no seam gets around that problem very clean there's an interesting conflict over forward-facing seams in the main floor of the rover it involves the seam between the main floor and the rear floor assembly here's a close view of that seam being welded with the car pointing to the left the main floor assembly which is nearer to us sits on top of the rear assembly so that the weight of the passenger on the floor will get structural support from the whole seam not just the welds but this produces a forward-facing seam in one of the most vulnerable places of all underneath the car where it's exposed to the full force of chippings mud water and salt so the center floor section shown here in yellow is designed with a bend or recess so the seam is concealed when viewed from the front and therefore protected but some problems virtually defy solution because we want windows that wind open it's impossible to seal the area around the window so if some water is bound to get in how can the designer minimize the damage well here's the inner half of a door the thickness of the door is very important because panels that are close together will trap condensation drainage holes are provided along the bottom making sure of course that water draining away doesn't run over any seams so it's very important that these drainage holes are kept clear otherwise water can accumulate in the bottom of the door shortly followed by a nice healthy patch of rust and this particular door also shows an interesting design fault for most of the door the inner and outer panels have been kept well spaced but just in the bottom corner they're only a couple of millimeters apart it's a stagnant area condensation and poor drainage have resulted in severe corrosion careful design can be seen at every stage of car production this is the assembly line where the side of the car begins to take shape wherever the seam between two components is especially vulnerable another trick can be used adhesive can be applied to seal the seam but it's vital that the adhesive used is compatible with the oil that still covers the components this is effectively the beginning of the pipe leading to the fuel tank with the adhesive helping the seal the first of several spot welds is applied just like the door the metro tailgate has two panels metal to metal adhesive is used to help stick and seal the two halves together [Applause] one of the main objectives of the accelerated corrosion program is to identify potential sites of corrosion within seams it's where we've got an inconsistent ring of sail around this yeah i'll just give you a gap at the bottom there you get the water leak coming straight that's right i think the leak goes down into here we've got a problem the results of this test may well mean a change back on the production line [Applause] as well as metal to metal adhesive door construction involves a different type of join a clinched seam clinching involves folding the outer panel right over the inner one to make a strong seam reinforced by the adhesive but we now can't get paint inside this seam so subsequent treatment will be needed if corrosion is to be avoided of course all seams on a car are potentially vulnerable here on the main rover line several sub-assemblies are coming together already this represents over 400 components that's a lot of scenes so why are they using steel in the first place well cost is a major factor an all stainless steel car for instance would be very expensive and also difficult to make and there's a special problem if we use different metals in combination metals can be arranged into a galvanic series metals further down the series are preferentially attacked by metals higher up so if stainless steel and normal steel are joined together the normal steel corrodes at the join but if steel and aluminium are connected it's the aluminium which will corrode see if you can identify the metals involved in these examples of galvanic corrosion a clue one of the metals is always steel but there's another type of corrosion to consider the earth return for this side light ran via the body of the car and the zinc alloy casting on the back has suffered not only from galvanic corrosion but also from stray current corrosion two wires should have been used so that the current doesn't flow through the casting so mixing metals has its problems and there are still good reasons for making most cars out of steel we must consider therefore how our steel car can best be protected from the effects of corrosion the body is now complete and these cars still rather oily are about to be cleaned using detergent then washed with deionized water before going into a phosphate bath this contains a complex mixture of chemicals but is based on phosphoric acid when the cars emerge the bodywork doesn't look dramatically different but a complex surface phosphate of iron and zinc has been produced sophisticated computer control of the phosphating process is necessary to get the surface just right with strong adhesion to the metal after phosphating the cars are dried ready for the next step which is electro coating this is the most important step in the whole process of protection these heavy-duty electrodes will carry up to 400 volts dc making the whole car body cathartic two electrodes are used simply to give a good connection the tank containing the electrocode forms the other electrode the electrocoat is a complex mixture of organic resins because the important resin species are cations they are electro deposited onto the phosphated steel body forming an extremely tough uniform coating a major advantage of this process is that because the coating is electro deposited it extends to fairly inaccessible areas after being baked in an oven the first of the paint stages begins the severest attack on a car is underneath and so a comprehensive underseal must be applied this is a pvc polyurethane mixture which is both tough and flexible it is abrasion resistant and also has anti-chip properties during the painting stages a lot of attention is also applied to sealing seams within the wheel arch is an inaccessible seam that's liable to trap water the result can be very painful for the car owner so what's to be done these so-called flow sticks are thermoplastic bitumen-based compounds dropped into the right position when the subsequent paint coat is oven baked the flow stick will melt and run down thus sealing the seam but there are many more seams that need to be sealed and a veritable army of people descends on the car those clinched seams around the door will need sealing so will the ones around the tailgate one practical advantage with clinched seams is that they provide a ridge which guides the application of the sealant the sealant is pvc based and needs to stick well and also to flex slightly so that it won't crack when the car body is stressed once the seams are sealed the car is finally ready for some cosmetic treatment most of the undercoat is applied by robots with only the finishing touches being done by hand [Music] the main purpose of the undercoat is to provide a good base for the final color coat unlike the electrocoat which exactly follows the contours of the metal the undercoat flows into surface blemishes thus giving a smooth finish each paint coat is baked on which also serves to cure all the seam sealants although the top coat is primarily cosmetic it combines with the undercoat and electrocoat to produce a total protective coating for the steel but contrary to popular belief paint is not a barrier to water rather it serves as a tough mechanical protection for the underlying electrocoat and it's that which is impermeable to water however the effectiveness of all this protection depends enormously on how we treat the car when the paint is chipped and then left unattended rust will start but the main corrosion takes place not where the steel is exposed but in the steel around the chipped area underneath the paintwork where there's less oxygen consequently if left untreated the area of rust can spread rapidly as more and more paint is forced up away from the steel but rubbing salt into the wounds really speeds things up this test plate underwent the 12-week corrosion program including the salt spray this plate underwent the same test but without the salt [Music] and high temperature combined with high humidity speeds up the corrosion process greatly your car corrodes much faster during the summer when the temperature is high than it does in winter this chamber subjects the car to 100 humidity and a temperature that cycles between 42 and 48 degrees centigrade every hour so parking your wet car in a warm and ventilated garage will cause rapid corrosion but if external corrosion is still up to the owner the problem of corrosion inside the body work has largely been solved all potentially vulnerable seams and joints receive further treatment wax injection injected into all the cavities in the car this seals any remaining seams very effectively in fact after the electrocoat wax injection is the most important single factor in preventing corrosion all the anti-corrosion techniques currently used by austin rover are certainly producing better cars the present measures mean that austin rover cars now come with a six-year warranty against corrosion [Music] some things though are still up to you it's important to wash wheel arches sills and underneath the car

Channel: Clive Davis

Views: 2,498

Rating: 5 out of 5

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Id: 3tB9nef5HTA

Length: 22min 31sec (1351 seconds)

Published: Wed Jan 06 2021