Why Do Corroded Corrugated Iron Roofs Have a Striped Appearance ?

Why do corroded corrugated iron roofs have a striped appearance ?

Dirk HR Spennemann

Techniques in Historic Preservation

Why do corroded corrugated iron roofs have a striped appearance ?

Dirk HR Spennemann

Albury November 2015

Dirk HR Spennemann, Why do corroded iron roofs have a striped appearance?

© 2015. All rights reserved. The contents of this publication are copyright in all countries subscribing to the Berne Convention. No parts of this report may be reproduced in any form or by any means, electronic or mechanical, in existence or to be invented, including photocopying, recording or by any information storage and retrieval system, without the written permission of the authors, except where permitted by law.

Cover: Photograph © Dirk HR Spennemann 2015

Preferred citation of this Report Spennemann, Dirk HR (2015) Techniques in Historic Preservation: Why do corroded corrugated iron roofs have a striped appearance ? Institute for Land, Water and Society Report nº 93. Albury, NSW: Institute for Land, Water and Society, Charles Sturt University. ii, 17 pp.; ISBN 978-1-86-467276-3.

Disclaimer The views expressed in this report are solely the author’s and do not necessarily reflect the views of Charles Sturt University.

Contact Associate Professor Dirk HR Spennemann, MA, PhD, MICOMOS, APF Institute for Land, Water and Society, Charles Sturt University, PO Box 789, Albury NSW 2640, Aus- tralia. email: [email protected]

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Contents

Background ...... 1 The Production of corrugated galvanised iron ...... 1 Galvanised Iron ...... 1 The hotdipping process of zinc coating iron ...... 2 Corrugated Iron ...... 7 Arrangement Pattern of the Sheets ...... 9 Corrosion Processes (simplified) ...... 10 Corrosion of corrugated iron ...... 12 References ...... 15

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— iv — Background Historic corrugated iron, some of which is over 100 years old, poses a range of conservation challenges to heritage managers and property owners.1 Many of the roofs are corroding with an increasing tendency to leak. Rusty iron roofs are a common sight in many rural areas and older industrial estates. It is frequently asserted in heritage literature that the striped appearance of many historic roofs (e.g. Fig. 1) is caused by sheets that were laid in an alternating fashion.2 The following quote may serve as a representative example: “The single-sheet system [of galvanising, ed.] often produced uneven coatings and generally the coating was thicker on one surface than the other. The common practice of laying alternate sheets upside down (to save on overlap, if not watertightness) has left us with many old roofs with a striped appearance resulting from the more extensive rusting of the more thinly coated surface” (NSW Heritage Office, 1998). To examine the nature of corrosion of corrugated iron roofs, this document first ex- amines the hot-dipping process to zinc coat iron products from its invention by Stanislas Sorel in 1837 and traces the manufacturing method in the nineteenth century. While the hand dipping, and later the machine dipping process (see below), tended to result in une- venly coated sheets, the above stated reasoning is flawed on several fronts: the assumption that one side of the sheet metal will consistently be thinner coated than the other during the hot dip process; and that the sheets of flat iron are consistently fed through the rollers of the corrugating machine with the same side up. The second part of the study explains the principles of corrosion that cause the phenomenon why corroded corrugated iron roofs have a striped appearance. As will be shown, the striped appearance is caused by differential corrosion, the processes of which will be explained in this technical note.

The production of corrugated galvanised iron During the first half of the nineteenth century two technological developments occurred that gave rise to corrugated galvanised iron: the discovery that corrugated metal sheets had a much greater strength and load bearing capacity than flat sheets, and the discovery of a commercially viable process to make iron more rust resistant by coating it with zinc.

Galvanised Iron During the mid-nineteenth century, galvanisation (the application of zinc on ferrous metals) became a wide spread method of slowing down the corrosion of products made from sheet iron. True galvanisation is the electrochemical coating of one metal with another. While these processes were developed in the second half of the nineteenth century, they

1 . For general, by-and-large unreferenced overviews on corrugated iron and its conservation in Australia see the publications by the various state heritage offices (Heritage Branch [Qld], 2014; Heritage South Australia, 1999; Heritage Victoria, 2001; NSW Heritage Office, 1998; WA State Heritage Office, 2013) as well as guidelines by lo- cal councils (Brisbane City Council, 2014; Brooks & Loveys, 2014).—see also Warr (1992); Wright (2000); Historic Scotland (2008); and the General Services Administration (2014).—For corrugated iron see also Gayle, Look, and Waite (1992) and Walker, McGregor, and Stark (2004).—For recording of historic corrugated iron see Spennemann (2015e). 2 This study forms part of broader research focus into rural vernacular architecture in south-eastern Australia (Spennemann, 2015a, 2015b) and the use and preservation of corrugated iron (Spennemann, 2015c, 2015d, 2015e). Dirk HR Spennemann, Why do corroded iron roofs have a striped appearance? required ready access to electricity and were not really cost effective for large volume production of commonplace objects. In addition, the coating was only thin and thus not suita- ble for sheet iron for construction purposes (Downs, 1976). Setting aside true electro-galvanic processes, the common process until the end of the nineteenth century was hot-dipping, that is dip (at first hand-dip and later machine-dip) the iron sheets in a pot of molten zinc (at about 450°C)(Davies, 1899, p. 60ff). Even though utterly misleading, by the early 1860s the usage of the term ‘galvanised iron’ had become so widespread that it was retained (Strauss et al., 1864, p. 48) even though all zinc- coated iron, and later zinc-coated steel, was produced by the hot dipping process or a ver- sion thereof.3

Fig. 1. Example of a corroding corrugated iron roof, showing the ‘classic’ striped appearance (54 Wallace Street, Holbrook, NSW).

The hot-dipping process of zinc coating iron The original patent application filed by Stanislas Sorel in France, and soon after in the USA, included five versions of hot and cold galvanising of iron (Ledru & Sorel, 1837; Sorel, 1837). Commercialisation of the hot-dipping process soon commenced in France and other countries, aided by the fact that minor modifications to the Sorel’s process al- lowed other manufacturers to circumvent the patent (Spennemann, in prep). The original patent application filed by Stanislas Sorel included the following description of the hot-dipping process that, with minor variations, remained fundamentally the same until mechanisation in the late 1900s: The first process-that of coating the articles to be protected ·with metallic zinc-is to be effected much in the same manner in which tinning is performed: that is to say, the articles to be coat-

3 . Hot-dipped galvanised iron shows a surface colour with a characteristic broken pattern, the ‘spangle’ (GalvInfo Center, 2015).

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ed must be rendered clean and free from oxide by processes analogous to those followed in preparing them for ordinary tinning, such as immersing them in diluted sulphoric or muriatic acid, scouring them, &c which processes, being well known, need not be described. The zinc in like manner must be poured in proper crucibles or other convenient vessels adapted to the nature and size of articles to be operated upon, special care being taken to keep the metal covered with sal-ammoniac or other proper flux, and to regulate the heat in such way as is required by the volatile nature of the metal. The articles to be coated, after being dipped into the melted zinc, are to be withdrawn slowly, that too much of the metal may not adhere to them. They are then to be thrown into cold water, rubbed with a sponge or brush, and dried as quickly as possible, as otherwise they may be injured by the appearance of dark spots, which it is desirable to avoid” (Sorel, 1837). Little changed on a fundamental level in the years thereafter. An illustration in a French illustrated paper shows the process of galvanisation of various objects (Fig. 2).

Fig. 2. French illustration of 1859 showing the process of galvanisation (Vauvert, 1859) The hot-dipping process as practiced in the mid-nineteenth century can be gleaned from an 1864 description of Frederick Braby and Co’s Fitzroy Zinc and Galvanised Iron Works (Euston Road, London). It is surprisingly similar to the original patent application: [In the galvanizing room]…we see numerous wood and slate tanks, and a large wrought-iron bath. Mr. Braby tells us that the iron sheets and other articles which it is intended to galva- nize, are first immersed for several hours in a bath of diluted hydrochloric or sulphuric acid, or a mixture of both. They are then thrown into cold water, and taken out one at the time, to be scrubbed and scoured to detach the scales and thoroughly clean the surface. After this they are thrown for a few minutes into a bath of pure acid, then dried in a hot closet, and finally dipped in the large wrought-iron bath, which contains about 20 tons of molten zinc, covered with a thick layer of sal ammoniac; from this they are slowly raised, to allow the superfluous zinc to drain off, and the sheets are then passed through plain iron rollers, and afterwards

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bundled into 1 cwts., ready for the market. Other galvanized articles are simply thrown into cold water and then wiped dry. The fire all round the galvanizing bath is never permitted to go out” (Strauss et al., 1864, pp. 48–49). The hot-dipping process stayed the same for a considerable period time. In 1874 it is described as follows: “The iron article is dipped in dilute acid, hydrochloric, sulphuric or nitric, so as to expose a fresh metallic surface, then thoroughly washed and brushed, and immersed in a bath of melted zinc. When the surplus zinc has run off the article is cooled, and, if necessary, dressed. Some- times thick plates are heated in a furnace before dipping, and hammered to remove the scale of oxide which is produced” (Anonymous, 1874).

Fig. 3. Frederick Barby’s Galvanised Corrugated Iron workshop in 1863.4

James Davies, in his end of century treatise on the British galvanised iron industry, described the by then obsolete process as follows: When this trade was first established the sheets were universally galvanized by the "dipping process," no machinery whatever being used. The bath was generally 8 feet 6 inches, or 9 feet 6 inches, long by 2 feet wide by 4 feet deep, and would contain twice the amount of spelter of the baths of to-day. A bar of T-iron, upon which an iron plate was riveted, was placed on the bath longitudinally. This plate was just deep enough to go into the metal, when the bath was at its lowest working height. Its object was to divide the flux, so as to keep the flux on the exit side in the best condition. A dipper and under-hand were employed at the front of the bath, who jointly plunged the sheet into the metal and by means of their rods passed it under the bar, bringing it up through the flux on the other side. It was then seized by tongs held by the " takers-out," of whom two were employed, who gradually drew the sheet out of the metal, and when the surface was "set,’ or crystallized, plunged it into the water, it being passed afterwards to the sawdust boxes to be dried. If the sheet was required to be " bright " galva-

4. Reproduced from Richards (1992, p. 29).—The image originally appeared in The Illustrated News of the World of 31 October 1863. It was then reproduced in an unidentified Australian paper, a clipping of which was reproduced without attribution) by Richards.—Attempts have been made to source a copy of the original 1863 publication from the British Library, then only library that holds copies of the The Illustrated News of the World of the relevant date. At the time of writing, the overseas request for digital reproduction had not been filled.

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nized (i. e. not crystallized), it was plunged into the water before the crystals had begun to form. If the sheets were large, the two " takers-out " would be employed in the operation, and if the sheets were longer than the bath they would be doubled. By this process it was requisite that the sheets were carefully dried before entering the bath, as the least damp would cause the metal to fly. The process is objectionable in every way, as the coating is thicker and lacking in uniformity, and it is entirely dependent upon the skill of the men employed in taking out. In addition, there is a great consumption of muriate of ammonia, as the whole surface of the bath requires to be covered. The sheets are also liable to damage by reason of the accumula- tion of the dross at the bottom of the bath. When the dross is high the sheets have to be forced through it, often causing damage to thin sheets. This process for sheet galvanizing is now entirely dispensed with amongst English galvanizers, but I have seen it at work during recent years by some Continental makers” (Davies, 1899, pp. 62-63). As is evident, hot-dipped iron often acquired a thicker than necessary coating of zinc. While not only expensive and wasteful of zinc, too thickly coated zinc tended to spall off when the iron was bent. To prevent this and to ensure a more even coating, the sheet could be sent through rollers suspended in the molten zinc, with excess zinc brushed off with rollers the sheet left the bath, a process that had been invented as early as 1848 (Anonymous, 1848), but which became prominent by the end of the nineteenth century (Flanders, Calder, & Foster, 1916, pp. 73–74; Vogel, 1895). The above-cited James Davies describes the process as follows: “The process in vogue by firms who produce a good quality economically, is as follows: A strong, square iron frame is made, into which two rollers work. These rolls are made of the best hammered forgings, and are turned. The usual size is 3 feet 6 inches long, by 9 or 10 inches diameter. This frame and rolls are suspended from bars placed over the bath, and at such a depth that the rolls are completely immersed in the metal, and the "bite" of the rolls 14 to 15 inches below the surface. The rolls have a hammered wrought iron pinion on the end of one, and two wrought-iron pinions at the other end, and are driven by another wrought- iron cog-wheel on a shaft placed on the side on the bath casing. The pinions and cog-wheel should be cut with teeth as large as practicable. The shaft has a fly-wheel and a four-cone pulley, and a corresponding cone-pulley on the overhead motion for varying the speed according to the thickness of the sheets. A thin sheet takes less time to coat than a thick one. It is advisa- ble (though not universally adopted) to have a pair of rolls of 6 inches diameter at the entrance side of the bath to guide the sheets into the flux-box, its only object being to relieve the labour of the dipper. A flux-box is placed on the entrance side and one also on the exit side of the bath. From these flux-boxes simple guides are arranged, which are removed when the day's work is finished. - Every competent dipper knows how these should be arranged, and he generally places them according to his own ideas of working. The sheet goes into the first flux- box in the wet state, and as it emerges from the flux-box on the other side it is seized by a boy armed with a pair of self-acting tongs, which are attached to a rope running over a pulley. The boy raises the sheet in a vertical position, and, when quite clear of the metal, by an adroit movement he rolls it up and plunges it into the water. A mechanical movement can be arranged with a clutch to wind the rope, and so ease the boy's labour. Two water "boshes " are used, placed side by side, and as the water in the first gets hot rapidly, the sheets are plunged into the second, to prevent them from drying white before they are saw dusted. In addition to this the hot water readily dissolves any particles of flux that may have adhered to the surface of the sheet. From the water they are passed to girls, who rapidly brush them in a trough con- taining sawdust, finally drying them singly over an open coke fire. It is essential to pass the sheets through a pair of 6 inch rolls after leaving the bath to ensure the corners and edges being level preparatory to the corrugating process.

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Fig. 4. Production flow-chart of nineteenth century manufacture of galvanised corrugated iron

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This process is the best, and the one I worked out so successfully for many years when en- gaged in the manufacture. The action of the rolls in the bath keeps up a constant circulation of the metal, and the dross is not so likely to get solidified as by any of the process” (Davies, 1899, pp. 64–66). In 1888 John Lysaght Ltd developed a four-roll galvanising machine that ensured an even more uniform coating and thus provided reliable quality, while at the same time sav- ing on zinc. Galvanised iron produced by this process shows a homogenous surface colour. Given the general technique of dipping individual pieces and sending them through rollers, there is no logical reason to assume that one side should be coated with more zinc than another. What can be expected in a manual hot-dipping process, however, as was described for Frederick Braby (see above), is that the end of the sheet that was pointing downward in the dripping off process during cooling will have a slightly thicker coating of zinc than the upper end.5

Fig. 5. Various machine designs for the production or corrugated iron (Anonymous, 1880).

Corrugated Iron Corrugated iron, i.e. flat sheet-iron that has been cold rolled into a wavy surface, was first patented in the United Kingdom in 1829 (Palmer, 1829). A factory was established by Richard Walker in 1830 (Dickinson, 1944). Many manufacturers quickly offered the product once the original patent had run out in 1843, even though Palmer tried to continue pro- tecting his interest with variations to the original patent (Palmer, 1842). A wide range of

5 This is caused by the fact that the surplus liquid zinc running down the sheet cools as it drips, with small amounts adhering to the surface rather then running off. Nineteenth century rollers as well as the modern con- tinuous hot-dip lines avoid this.

— 7 — Dirk HR Spennemann, Why do corroded iron roofs have a striped appearance? techniques6 and machines were developed, from hand rollers to steam-driven rollers and stampers used by the larger manufacturers (Fig. 5) (Anonymous, 1880, 1884). By the 1850s, the use of corrugated iron had become wide-spread. It served mainly as a roofing material, given the structural strength it provides along the long axis of the corrugations, and later as weight-bearing iron also for suspended ceilings. During the second part of the nineteenth century, Australia became the premier export market for the British corrugated and galvanised iron industry. Indeed, it has been posited that corrugated iron has a special place in Australian architecture and aesthetic (Lewis, 1982; Meyers, 1981).

Fig. 6. A side-way’s corrugation machine in operation at Lysaght’s Kembla works 1938 or 1939 (Lysaght Ltd Pty, 1939, pp. 102–103).

6 . See a number of pre 1850s patents for corrugated iron (Brown, 1847; Spencer, 1844) for roofing (Malins, 1844, 1845; Spencer, 1844) and fire proof floors (Porter, 1848).

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Setting aside stamping, which later on was primarily used for creating deep and angu- lar profiles, corrugation was achieved by feeding flat sheets of iron into a series of rollers that formed the surface of the iron sheet into the sinoid curves. Two principal methods exist. Feeding the sheet at the short end, thus creating the corrugations as the sheet went lengthwise through the machine.7 This process was invented in 1844 by John Spencer (1844) of the Phoenix iron Works, West Bromwich (Dickinson, 1944). The second method was to feed the sheet at the long end, thereby creating the corrugations as the sheet went sideways through the machine (Fig. 6). This process was invented in 1845 by Edmund Morewood and George Rogers (1845). In the lengthwise process, the iron is folded into the corrugations, the width of the sheet decreases and the stresses on the sheet increase. Longitudinally corrugating machines have a top and bottom set of several single wheel rollers set at the required spacing, or solid rollers that have a fixed spacing.8 As the sheet is fed through, all corrugations are being created at once, setting up substantial pres- sures in the material. Longitudinally corrugating machines were suited for the softer sheets made from puddled iron, but unsuitable to corrugate the harder steel sheets. Sideways corrugating machines, on the other hand, are comprised of two rollers that have the sinoid curves as ridges on their surfaces. Like longitudinal cogwheels, they inter- link and pull a sheet through (Fig. 6). In the process, only vertical pressure is exerted in the sheet, which is limited to the point where the rollers meet, thus creating one corrugation at a time. This process not only reduced stresses in the material, but also allows to corrugate the stronger steel sheets.

Arrangement Pattern of the Sheets The historic building literature discusses a variety of methods to overlap the sheets of corrugated iron. A six-inch long end overlap, and a single- (Globe Iron Roofing and Corrugating Co, 1890; McM, 1946, p. 32) or double-corrugation side lap is recommended in the historic construction literature (Branne, 1929, p. 601). This meant, however, that when covering a roof with 26" wide sheets, and a double corrugation side overlap, the sheets only yielded an actual coverage of 24". In addition, a 6- inch end lap and 2 corrugations-wide side lap added 25% weight to the corrugated iron sheeting resting on the roof trusses (Gillette, 1907, p. 531). Many farmers and other build- ers chose to maximise the coverage of their roofs at the expense of strength. The method covering the largest area with the smallest number of sheets in the half- lap (Fig. 7). This entails that the sheets are laid alternatingly with the edges of the corrugations pointing down and pointing up. Half laps, however, are not very watertight. In a survey of American barns, Hodges (1951) found that 1½ corrugation was the most common form or side overlap, followed by side overlaps of a single corrugation. Depending on the slope, a 1½ corrugation was deemed insufficiently watertight.

7 . See Fig. 3 (at right margin) for such a machine. 8 . A number of manual corrugated iron rollers are held in a number of museums and also depicted on the WWW (e.g. Peripetus, 2009). These are not machines for creating corrugated iron sheets as commonly asserted, but are devices designed to curve existing corrugated sheets to create the curved sections of bull-nose verandahs and corrugated iron tanks (Powerhouse Museum, 2015).

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Fig. 7. Methods of overlapping sheets of corrugated iron. The greater the overlap, the less roof surface is covered with the same number of sheets.

Corrosion Processes (simplified) Corrosion is a process in which a high-energy (processed) metal or metal alloy oxidises and returns to a low energy state similar to that of metal ore. Differences in the crystal structure of pieces of metal set up a voltaic cell. This can occur within a single sheet of metal, or between two sheets that are in contact with each other.

Fig. 8. Basic corrosion process of iron. At the anode, solid iron is oxidized to Fe2+and enters the electrolytic solution. The two freed, negatively charged elec- trons (e–) are transferred from the anode to the cathode through the electrically conductive metal(s). At the cathode, oxygen from the air is reduced to hydroxide ions (OH–). The Fe2+ in the electrolytic solution reacts with the hydroxide ions to precipitate Fe(OH)2 which oxidises with atmospheric oxygen, forming red-brown hydrated ion oxide (Fe2O3·xH2O) (adapted from Larsen, 2015).

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Electrochemically, corrosion is the transport of ions from the anode to the cathode, in the process freeing metal atoms that can subsequently bind with oxygen (Fig. 8). For corrosion to function, there needs to be one or more metals, an electrolyte (salts in solution), and oxygen. The corrosion of a range of metals, such as copper, aluminium or silver, creates a thin metal oxide film on the surface, which prevents further oxidisation as it forms an impenetrable barrier. The corrosion product of iron (Fe2O3), however, hydrates easily and thereby continually flakes off to expose a fresh metal surface. The speed of the corrosion process depends on the concentration of the salts in solution,9 the nature of the metals corroding and temperature. Higher temperatures enhance the chemical corrosion process (Hackerman, 1952; McNeill & Edwards, 2000).

Fig. 9. Schematic distribution of anodes and Fig. 10. Schematic distribution of an anode and cathodes on a uniformly corroding metal surface cathode on a localised corroding metal surface

Historic metals contain a number of impurities, all of which provide for breaks in the lattice structure of the crystallised iron. These impurities and lattice defects tend to become the anode in the electrochemical cell. In the case of uniform corrosion, there are many anodes and cathodes scattered on the surface (Fig. 9). Instances may occur where the anode is highly localised while the remainder of the surface acts as a large cathode (Fig. 10). This can be the case if a protective layer (paint, galvanisation) is pierced in small area while the rest of the metal retains its protective coating. It also occurs as part of galvanic corrosion, when a smaller element is made from a different material than the rest. If iron is in direct contact with other metals, the different galvanic potential of the metals comes into play, whereby some materials are more electropositive than others. A simplified galvanic series for the metals commonly found in the major architectural components is as follows: [+] stainless steel → bronze → brass → tin → lead → cast iron → rolled iron → carbon steel → zinc [–] The more electronegative [–] a metal, the more likely it will act as the anode, while the more electropositive [+] material will act as the cathode (Francis, 2000). This implies that if iron is in physical contact with a brass or bronze alloy, it will decay much more rapidly than if it were only in contact with other iron. The effects can be quite dramatic (Fig. 11).

9 . There are sufficient air-borne salts to facilitate corrosion as long as moisture is present. Of course, higher airborne salt concentrations, such as close to the sea, accelerate the electrochemical process. Corrosion of historic metals is accelerated in marine environments due to the increased concentration of airborne salts (Look & Spennemann, 1996) especially where vegetation cover provides as prolonged moist environment (Spennemann & Look, 2006).

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Galvanization, i.e. the zinc coating of iron, makes use of this process inasmuch the zinc coating will decay in favour of the iron. As oxidised zinc, like many other metals, forms an impenetrable barrier, however, galvanised iron products tend to have a long life expectancy. If the oxidised zinc coating, however, is subjected to mechanical impact, ranging from raindrops and hail pellets, to treading on the roof, fresh, as yet oxidised metal will be exposed, which, once oxidised, will recreate the coating. Thus, the zinc coating will gradually erode over time, eventually exposing the underly- ing iron, which will then commence to corrode itself. The fact that an oxidised zinc coating forms an impenetrable barrier is also the reason why the undersides of roofing sheets often appear to be in near pristine condition, while the exposed (upper) sides are heavily corroded.

Fig. 11. Effect of galvanic corrosion on part of the elevation gear of a Japanese 120mm dual-purpose gun on Kiska (Aleutian Islands).10

Corrosion of corrugated iron Tests showed that nineteenth century sheet iron produced in the hot-dipping process, when cold rolled into corrugated iron, was more susceptible to failures of the zinc coating than electroplated galvanised iron (Anonymous, 1905; Vogel, 1895). The coating would crack and spall at the stress points. This suggests that on a given sheet of corrugated iron, the zinc coating on the rests and valleys will corrode faster than on the slopes. As the galvanic series (see above) shows, low energy iron, such as cast iron, is less likely to corrode than high-energy iron, such as carbon steel. The same applies to iron that has been mechanically manipulated. Wrought iron will corrode faster, especially at those locations that are bent/twisted as the lattice structure has been damaged. This also applies to flat sheet iron that has been run through rollers or stampers. The bent areas of the crests and valleys are more likely to have damaged lattices and thus are more prone to form anodes than the adjacent unbent sloping sections (Fig. 12a).

10 . Image source: Spennemann (2008, Fig. 72).—For background see Spennemann (2011) and Spennemann and Clemens (2014).

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Fig. 12. Differential stresses in a rolled sheet of corrugated iron. The bent areas of the crests and valleys are more susceptible to corrosion As mentioned above, once the surface covering of zinc has corroded, uniform corrosion will start to set in. The ridges and valleys are likely to form anodes, as these are both the worked/bent areas; the ridges are also the surfaces subject to greater physical impact and abrasion (people, objects, scraping branches etc.) (Fig. 13a, Fig. 15a). As corrosion progresses, the entire ridge will form an anode while the slopes and the valley will act as the cathode (Fig. 13b, Fig. 15b–c). Over time, the valleys too will act as cathodes (Fig. 15d). This process gives the sheets a striped appearance. If this were a single, isolated sheet, then eventually the entire surface would be corroding, with ridges and valleys continuing as anodes until ‘rusted through’. In situations where sheets are overlapping, the ongoing corrosion process will eventually reach a tipping point where an entire sheet acts as the cathode and an entire sheet acts as the anode (Fig. 13c). This then gives the roof the broad-scale striped appearance (see Fig. 1).

Fig. 13. Corrosion process of adjoining sheets of corrugated iron (for description see text). In addition to inherent stresses in the metal, impact damage can alter the lattice structure and therefore create conditions favourable to the development of an anode. Ex- amples of these are nail and screw holes that penetrate the iron. If the holes are small and clean, then the covering zinc may form a protective oxidised layer.

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These nail holes allow for crevice corrosion, both at the overlap at the sides and at the end (Fig. 14). This is caused by capillary action sucking in, and subsequently trapping, moisture in the interstices between the two sheets (Hodges, 1951, p. 39 ff). The same occurs at the sides. Wind pressure on moist roofs will exacerbate the effects and drive moisture further into the laps than possible through mere capillary action.

Fig. 14. Principles of crevice corrosion

a b

c d Fig. 15. Progressive corrosion of single corrugated iron sheets. Note the development of the anodes on the crests of the corrugations..

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