catherine houska: my presentation today wassponsored by the nickel institute and imoa, which are two stainless steel industry associations.and if you haven't found it in your handout, there is a flash drive with over 250 documentson it to help you with specification of stainless steel. so i did want to bring that to yourattention. as a metallurgical engineer and material scientist, the way i look at problemsis a bit different from many of you in the room. so i wanted to put the arch's stainlesssteel in perspective. when the arch was constructed, stainless steel had only been available for50 years. depending on whether you credit the brits or the germans, it was either inventedin 1912 or 1913. the structural applications for stainless steel had only been around forabout 20 years, with the largest of which
had been rebar for the progreso pier in mexicoin the 1940s. but otherwise, as a structural material by itself, there had only been fairlysmall applications and very little research. so when the arch was designed using stainlesssteel plate as an important part of the structural element, as was explained, there was concern.and so a great deal of research was done by the stainless steel industry, starting inthe early 1950s through the 1960s. that then led to the first standard in 1968 which wasa-i-s-e a-i-s-i. and then that became the seiase 8. there has been subsequent researchover the years. there's a japanese standard, eurocode 3 has all structural sections, andnew aisc design guide 27 that came out about a year and a half ago, i was on the steeringcommittee for that. all of this research was
used first, not on the arch, but the unispherecan actually claim to be the first large structural application for stainless steel. the researchwas already in place. of course, the arch was already under construction. but it wasdonated by the stainless steel industry as essentially a piece of advertising. it hadstainless steel from every producer, 304 welded together and is still doing well. stainless steel has been used in many restorationprojects, one of the most recent i am familiar with is some work on the acropolis where they'reusing a duplex stainless steel for high strength. but it's very common to use it in hidden applications,not just in the very visible means that we see it on the arch. one example of that wasthe statue of liberty restoration. when you
put large sections of copper in direct contactwith cast iron in a structure where moisture is going to infiltrate, you're asking forgalvanic corrosion problems. the designers did understand that, provided for separation,which went away over time, and then there were additional efforts to separate the metals.but there was iron corrosion, which expanded, damaged the copper, and in the early 1980s,they started looking for solutions. the framing, the secondary framing is a duplex stainlesssteel, and then the bars that are in direct contact with the copper are 316, which hasthe same coefficient of thermal expansion as the copper, unlike the original cast iron. there have been many large building framingapplications for stainless steel. the earliest
were to get stainless steel into the structuraldesign guides around the world. the largest after that was the canadian national archives.there also have been since the gateway arch, many large welded structural stainless steelprojects. as steve mentioned, i have worked on many of these as a member of the designteam. when i worked on the us air force memorial, one of the first things i pointed out was,"take a look at the arch, there is no access. we need to provide a means of inspections."so there are loops going up each of these spires, if you happen to visit you will seethem. that's three-quarter inch thick, type 316l plate that was welded together. no carbonsteel. so i often get questions about what stainlesssteel is and how it works. stainless steel
is iron plus at least 10.5% chromium and islow carbon. the name, going back to the very beginning, was stain-less, not stain-free.just want to point that out, it's a rather critical point. you can add corrosion resistanceto stainless steel by adding more chromium, molybdenum, and nitrogen. and you can pickstainless steels that will not corrode in a specific environment, but you have to pickthe right one. so you can't just say stain-less. form-ability and weld-ability is added bynickel. so on the gateway arch, we have a nickel containing stainless steel. so thatmeant that if there was a welding problem, that weld could be cut out and re-welded,and it really wasn't a big deal which was important.
there are also inherent differences betweenstainless steels in corrosion resistance, and you can do a calculation based on thechemistry to determine the relative corrosion resistance. there's a lot more about thison your flash drives. and create a pren number, that's a pitting resistance equivalent number.i've shown a few stainless steels that are used for structural applications. and youcan see the last column is a pren number. the higher the number, the more corrosionresistant the stainless steel. you can see that 304 is at 18. 316, you're probably familiarwith, is at 25. as you go all the way up to those that are in the 40s. those are stainlesssteels that can be immersed in seawater and not corrode. so there is a wide range. thestrength of 304 to 316 is essentially the
same as that for carbon steel so in the designof the arch, we were well matched in terms of strength of materials. i showed duplexstainless steels that are sometimes used, as i showed in the statue of liberty, forhigher strength. they are about double the strength of the carbon and austenitic stainlesssteels. steve showed some pictures of the inside ofthe arch and the carbon steel with very little corrosion on it. as carbon steel corrodes,you get a layered appearance, and those layers actually tend to retain more moisture andaccelerate corrosion. stainless steel, when it corrodes, pits, so it's a very differentcorrosion process. there has been a lot of work done by metallurgists like myself, andi'm involved in three corrosion projects right
now. two in the middle east and one in china,where we're doing long term corrosion studies. but you can take that data, from whateverpart of the world it is, and use those corrosion rates to predict the life of materials. andthere's a lot of data available. i'm just showing this for standing seam roofs and it'squite easy to make those comparisons. so when we're looking at construction materials,any type, they all corrode, just as our bodies corrode. it's called aging, they oxidize.so we need to look at the environment. is there pollution? or was there over the structure'slife? what type? is there salt exposure? coastal or deicing, and at what level? what are theweather conditions? if you get regular, heavy rain, and you have a structure that's fullyexposed like the arch, then you get natural
rain cleaning. and at the top, with wind levels,we probably get power-washing levels of rain cleaning, as happens at the top of the chryslerbuilding, because that was measured as well. will there be maintenance? and are there aspectsof the design that might cause corrosion problems, like unsealed crevices? and then the typeof finish that you have also makes a big difference. mishandling, as was mentioned occurred a littlebit during installation, can also cause some surface contamination and damage. since steve talked a little bit about thestudies, and i don't want to infringe on what is going to be talked about later, i wantedto talk very briefly about what humidity means in terms of corrosion. if you do not havesalt in the environment, which you would not
have between the layers, then you need about80% relative humidity to have corrosion occur. and we have a time of wetness issue, a tow,which you might see if you're working with a metallurgist. and if you don't have a verylong period of time where there is moisture present, then you can't have corrosion. ifsalt is in the environment, and in this case i am looking at three salts that are bothfound in sea salt, but are also used for deicing. we used to just use sodium chloride, rocksalt, for deicing. and when we only had that in the environment, which wasn't introduceduntil after the arch was built, you had to hit 76% humidity and 50 degrees fahrenheitfor that salt to be actively corrosive. salts lower the humidity levels at which corrosioncan occur, but spring rains would wash most
of it away. calcium chloride, and unfortunately sometimesmagnesium chloride, are increasingly being used for deicing. when those are present inthe environment, then you only need 45 or 50% humidity and as soon as you're above freezing,they are actively corrosive. in the case of the arch, the highways are some distance away.but if you're working on a project where there are roads immediately surrounding it, findingout what a city or municipality is using is quite important. this is a 1980s us and canadiancorrosion map done by the automobile companies. ignore the west coast, because that's notparticularly accurate, but you can see in the 1980s, the effect of deicing salt andindustrial pollution around the whole great
lakes, well what was really the heavy industrybelt. i also show, on the right, data from the salt association, and the fact that wehave essentially doubled deicing salt use since that 1980s. and that has changed themap. if you're working on a coastal project, the us national atmospheric deposition programis a great source of data. this is just for one year, but you should look to see whatsalt exposure is for a location. for stainless steel, this stainless steelinspection, we were looking at a number of different things. first of all, a visual inspection.is there corrosion or other discoloration? the appearance of the welds, including possibledefects and any corrosion around them. corrosion patterns can tell you a great deal, as canthe appearance of a weld, and other visible
damage. we went through archival information,and we got five weld samples to evaluate. we used gsr, those are gun shot residue kits,tmr has been using them for a very long time, because you're able to collect fine particlesfrom the surface that you would not otherwise be able to collect to evaluate without havingto do anything destructive. just as the police can determine if someone has gunshot residueon their hands or collect fine fibers. you can use a scanning electron microscope todetermine exactly what those are, which is what we did. we also looked at the finish,which will be discussed later. and there were cleaning trials. so in the 1960s, the stainless steel camefrom two producers, there were 886 tons of
304 that came from what was then eastern stainless,which is now part of outokumpu and what was then a division of us steel, which is nowati alleghany ludlum. having friends at both places, i talk to them about technology atthat period. the first aod, argon oxygen decarburization furnace, in the united states was not installeduntil 1974. aods allow removal of a lot of impurities. it was not possible to make lowcarbon stainless steel consistently until those were introduced. so we had to assumethe stainless steel on the arch was not going to be low carbon, which introduces the possibilityof things like weld sensitization. it also meant it was likely to be high sulfur, andhigh sulfur also introduces some potential corrosion issues.
in order to understand what we found on thosegsr kits, it was important to look at what historically had been around the site. andhere you can see the highways and the distance from them. but we took a closer look, andi did want to point out if you're working specifically with stainless steel, there isa scoring system from imoa, that helps you determine which stainless steel might be appropriate.in the industry around the site over the last 50 years, so we had three coal-burning powerplants, a coke plant, several steel mills, a copper mill, a zinc refinery, chemical plants,an ammunition plant that apparently managed to blow itself up on a somewhat regular basis.and they're remnants of them. and some still there. if we go back to the 1985 data fromthe national atmospheric deposition program,
unfortunately they didn't map it prior tothat. but you can see acid rain was certainly part of the environment then. now, it's essentiallydisappearing, but it was a factor in the environment. so, here's some gsr kits, and i'm showinghow they're used by the police, and the ability to collect very fine particles. these arewhat some of those particles look like. you'll see the grays, browns, and kind of a littlebit of a reddish tint. the top picture is at higher heights, as is the base with someof the streaking that is coming down from the welds. so we took these samples to ansem and analyzed the particles. and here are a few pictures. one shows iron oxide witha little bit of chlorides, and some common soil contamination. the other is sodium chloride,obviously deicing salt. but this summarizes
what we found. there were very small amountsof deicing salt and very minute levels. far too small to ever picked up with anythingbut this type of test kit on most of the arch, but it's very, very small. which isn't surprising,because of the highways. higher concentrations, most of those deposits are fly ash, they'referrochrome, and iron and steel slag, which are exactly what you would expect from coalfired power plants, coke plants, steel mills, which were in the area during the life. there were also some scattered iron particles,which you can get those from manufacturing sites. some copper and copper zinc, thereis a copper mill. and lead, titanium, other things that match with the industry that hasbeen in the area. then a great deal of common
soil constituents: clay, sand, et cetera,silica. this is what the base looked like. or still looks like. there is some very light,superficial staining. i have a close up picture of a very small pit. this is tiny, and thisis all superficial, easily removed, absolutely no effect on structural integrity. there issome embedded carbon steel, both in the graffiti and there are these long lines at the basewhich are probably from a plow or something similar. those are probably the most deeplyembedding. that embedded iron really should be removed, at least from the deeper scratches,because you could continue to have some corrosion underneath it. these are the welds, or examples of some ofthe welds. they were all full penetration.
as i mentioned, we have five samples, allthree-quarters of an inch in diameter. there were no weld imperfections that caused anyconcern. yes, there was some sensitization which i'm showing a picture of here. sensitization,when it causes corrosion, causes a classic band beside each side of the weld. absolutelyno evidence of that anywhere on the arch. if it hasn't occurred in the 50 years priorto this when pollution levels were higher, then it's not going to occur in the futureunless we have a dramatic change in the environment. so, again, no concern. the welds, we did findrecords of them, of exactly what was done, including the cleaning. it was done priorto aws d1.6, a structural welding code, but it was all done to boiler code requirementsat the time so again, no concerns about what
was done. we did confirm that it was all 304,including the weld material. i did want to point out that there are a lot of resourcesif you're working with stainless steel, including those on your flash drive. and some conclusions. none of the weld imperfectionsare a concern. there were some, but nothing at all a problem. the weld sensitization hasnot caused corrosion, so again, no problems. the surface discoloration is superficial.it will have no effect, it's all from industrial pollutants, i think i made a joke once onsite that we should send the carbon steel industry a bill, or maybe the power plantsa bill. but the cleaning would remove discoloration, but it is not needed. the only thing i thoughtshould be removed was some of the embedded
iron. thank you.