CHAPTER 13FINISHING IRONWORK
Corrosion and its Prevention
Volumes have been written about corrosion and it's prevention and will not be repeated here. Instead I shall report only my observations relative to the blacksmith and his ironwork. Over the years blacksmiths have used a variety of coatings and home brew finishes on their Ironwork, most of which are acceptable for indoor use. However, when exposed to the elements or damp environments wrought ironwork must be carefully protected to prevent corrosion and it's eventual transition to dust or the scrap heap.
Most blacksmith's immediate concern is to satisfy the customer and not receive undue complaints about rusty iron during his lifetime. But, the odds are, that most architectural ironwork produced will out last the Smith and his customer and their children and probably their great-grand children. Serious thought and care should go into protecting work that may last for centuries. An understanding of the mechanism of corrosion and the peculiarities of forged ironwork is necessary to win the war against our common enemy, RUST.
A lot has been said over the years about the virtues of wrought iron over mild steel in Ironwork. Among these is wrought iron's resistance to rust. There are several factors involved here. The first of which is that the higher the carbon content of steel lowers it's resistance to oxidation. Wrought iron has virtually no carbon and therefor rusts less easily than carbon steels. The second factor is the fuel with which the iron was manufactured and worked. The majority of old iron work was produced using charcoal as fuel. Most modern ironwork has been produced using coal and mild steel. During the three thousand year history of blacksmithing charcoal was the smiths fuel. It's only in the last 100 years that coal has almost totally replaced charcoal as the blacksmiths fuel of choice.
Without KING COAL the industrial revolution would have stalled as we stripped every forest on earth to produce fuel for iron making. Without coal there would have been no materials or fuel for the great steam engines that powered the factories of the industrial revolution or the railroads that opened the American West and moved the materials of commerce world wide. Coal, alone, is the raw material of many chemical industries. Coal made possible the mass production of cheap mild steel and it's eventual replacement of expensive wrought iron. Steel manufactured with coal picks up sulfur and it is the sulfur that makes the big difference in corrosion resistance of old wrought iron as apposed to modern steel.
Using coal for producing forged ironwork leaves us with some rather special problems. In every coal fire there is a great range of temperatures and a variety of chemical reactions going on. At the edge of the fire it's just hot enough to liquify and then gasify the coal without burning it. A little further into the fire exists a carburizing atmosphere and at the center an oxidizing atmosphere. Many of the impurities in the coal, such as sulfur, exist in gaseous form in the fire and may combine with the hot iron to detrimental effect.
At the edge of the fire a cold piece of iron causes some of the vaporized fuel to condense on its surface. Plating the iron with coal and it's impurities. Every smith has noticed this shiny black area just outside the grey area that had been heated to forging temperature. Most of us have given it no great deal of thought or have tossed it off as a little bit of soot. But coal, unlike charcoal, is a very complicated mixture of various compounds including sulfur and several sulfur compounds that can be very corrosive. Coal deposited on iron is very difficult to remove. It's lubricity makes it difficult to wire brush off even when it is possible to get to all the affected surface with a powered wire wheel. This coating would not necessarily be detrimental if a uniform coating were achieved using low sulfur coal. Ironically some types of cast iron pipe is coated with a"bituminous" coating by bathing the pipe in liquified coal increasing the cast iron's natural resistance to corrosion. The problem facing the smith is that this coating is intermittent on his ironwork and more or less transitional. Paint applied over it will soon flake off or bubble up as the anhydrous coal compounds absorb water and form larger water bearing crystals. Then rain water or condensation gets under the paint and mixes with the sulfur bearing impurities and forms an acid that acts as a first class electrolyte dissolving the iron and combining it with the oxygen in the air. Soon there is a nicely pitted rust spot and another piece of ironwork is on it's way to oblivion. In the carburizing portion of the fire, when the iron or steel being worked is at a red heat, the chemically active iron absorbs carbon and sulfur from the coal fire. The carbon will increase the strength of a piece of low carbon steel, but the addition of sulphur makes mild steel brittle and ruins high carbon and alloy steels. In any case, the addition of carbon and especially sulphur increases the oxidation rate of iron and steel and is to be avoided. Using low sulphur fuel and avoiding long soaking heats is recommended. Gas forges burning natural gas or propane avoid both the sulphur problem and the coal plating.
In the oxidizing portion of the fire carbon is burned out of the iron and a coating of iron oxide forms on the work. The iron oxide coating is a fairly weather resistant finish for a short period of time, and holds up rather well for indoor use when oiled or waxed. However, the iron oxide coating is generally not very uniform and has a tendency to flake off when the iron is flexed or if the coating is too thick. Some portions of the coating will also contain sulphur from the carburizing portion of the fire. In all cases where ironwork is to be exposed to the weather the iron oxide scale should be removed.
Welding creates its own set of corrosion problems. Forge welding using a flux such as borax leaves an area coated with anhydrous borax. Over a period of time (weeks, months, or years) this coating will absorb water from the air causing the crystals grow in size appearing like a white fuzz on the metal. If the flux has been painted over it will still absorb water through the paint, expand, and flake off the paint. Arc welding with coated rods (electrodes) creates a similar situation or worse as the flux is often trapped in crevices in the welds. Arc welding fluxes start absorbing water almost immediately, and may continue to do so for years if not completely removed. After trying a variety of methods of finishing iron work I came to the realization that if its not clean there is no hope of the finish lasting. For interior work that you insist on waxing or oiling I recommend a thorough cleaning with a power wire brush. If you select a brush with a relatively fine wire (.018 - .020 ) the oxide coating will not be totally removed and the finish not hurt. Use an Oxyacetylene torch to burn off coal plating and to re-oxidize areas that are oxide free. Wire brush again to be sure that the remaining oxide coating is fixed and will not flake off. Wash the metal with soap and water to remove salt deposited from your hands and then oil or wax as soon as the water has dried. This is also the recommended practice if the work is to be finished with a clear lacquer. Whenever wax or oil finishes are used a responsible smith will explain to the customer how to care for and maintain the finish. I recommend printed care instructions if you use these finishes a great deal. For all exterior work there is only one practical solution, sandblasting. Take your work to a professional sandblaster and let him have at it! Sandblasting physically removes all scale, coal plating, and weld fluxes. It also gives the surface a slight "tooth" to help your finish to stick. Some people recommend chemical baths to clean iron work. So where do you get a tank that big and what do you do with the waste acid (now Toxic Industrial Waste)? Chemical cleaning may not remove some coal plating and the residual acid that may hide in joints is the worse thing you could have on your work. Chemical cleaning will also give the surface of the iron tooth, but not uniformly like sand blasting as the chemicals will have longer to work on clean places and little or no time on places plated with coal. Put your local sand blasting contractor to good use. After sand blasting, your work will be very clean but also highly susceptible to rust. A little light rust will not hurt but you should be prepared to paint your work immediately if not sooner! Your sandblaster may also help you with this as many sandblasters are in the industrial finishing business. Some government contracts require hot dip galvanizing (zinc dip) on exterior work. The problem with hot dip galvanizing is that it really screws up your surface texture and it must age for a year or so before it can be painted. What zinc does for iron and steel is that it acts as a substitute for the iron in the corrosion process. This is called anodic protection. In the process of sacrificing itself the zinc will plate a bare place on the iron and stop further corrosion. As it is practically impossible to keep a finish entirely chip free some type of anodic protection is a necessity for long term outdoor protection. The next best (or possibly better) first coat is a pure zinc powder paint. Sometimes called cold galvanizing, this is the type of paint that is used on the inside of your local water tower! The biggest draw back of zinc paint is that it has low chip resistance and is exactly the same color as your sand blasted iron. After applying a thin uniform coating of zinc paint a second coat of primer such as Dupont's Red Oxide Lacquer Primer should be sprayed on. The primer is stronger than the zinc paint and presents the proper surface for the finish coat. A finish coat of Black (or whatever) enamel maybe sprayed or brushed on, or a finish coat of lacquer sprayed on. I prefer Automotive paints as they are the highest quality product to be found anywhere and are designed to take constant sun and wind, salt spray and abrasives. After automotive paints come Machinery enamels. I've never tried any but I expect MIL-SPEC paints would be very good. A really first class finish is a two part epoxy finish. Epoxy finishes are very hard and take a great deal of wear and tear. Epoxy finishes are relatively expensive and should be applied by someone with the necessary equipment and experience with two part finishes. It has often been recommended to paint hidden surfaces prior to assembly. This is often impractical and could be counter productive if hot riveting or welding is to be used as an assembly technique. Also the problems of coal plating and other contamination are not addressed either. If heat is to be applied later to a prepainted joint the resultant burned paint and debris will create a worse situation than not painting in the first place. Any debris trapped in a hollow place will remain damp for long periods of time after exposure to water. The water and various compounds in the debris will act as an electrolyte and severe corrosion will result. I prefer to greatly thin zinc paint with lacquer thinner and turn the work so that the thinned paint can rundown into any hidden places. Several smiths I know avoid this problem by not using the commercially available hollow top rail and produce their own solid top rails. We could get completely paranoid on this subject and sandblast and completely finish a piece before it is assembled then refinish after assembly. If you are looking for immortality and really want your work to last forever use stainless steel. After heating and forging it looks exactly like forged mild steel. A wax finish can be applied and that freshly forged look will last practically forever. The Germans use a powdered spray technology to zinc plate their work. It is a highly dangerous process that OSHA would seriously frown upon here. I asked them why they did not use stainless steel to avoid the health hazards and was informed that "zinc plated steel lasts longer". This I seriously doubt. I expect the 10 to 1 cost difference between 304 stainless and mild steel had something to do with their opinion! Still, what cost is too great for immortality? Many years ago I forged aNorfolk latch from stainless steel for the front door of the old grist mill we live in. Even with salt from our hands and the unusually high humidity that rusts everything else overnight that latch looks exactly the way it did the day I installed it!
A lot has been said of the virtues of COR-TEN corrosion resistant steels. "Make it out of COR-TEN and let it rust" If you choose to use COR-TEN, remember that to retain its special qualities, you must use COR-TEN fasteners (Rivets, nuts, bolts, washers). All welding must be performed using special COR-TEN electrodes and the proper cleaning techniques, and that special precautions must be taken any where COR-TEN comes in contact with a foreign substance such as wood or concrete.
NOTE: Enamel may be applied over lacquer but lacquer will cause an enamel undercoat to boil up like applying paint remover. Lacquer drys almost instantly. Enamels get tacky in a few minutes, are dry to the touch in an hour or so, but should sit over night before being handled or moved. Most all primers are designed to help the finish coat stick and are not a rust preventive but a neutral ph coating.
CAUTION!: Popular hardware store paints such as "Rustolate" or "Zinc Yellow primer" are not satisfactory for painting architectural ironwork and do not prevent rust any better than any other enamel finish coat. Read the contents of the paint you use. Often a paint will declare "Zinc chromate rich" on the front of the can but the percentage of Zinc chromate will be less than 10% of the solids instead of the 75 or 85% that would be expected in a corrosion preventive paint or primer! Zinc chromate and cadmium based pigments do not provide adequate anodic protection in any case. Paints such as B-BQ Black are very good for stoves, trivets, and other hot use items, but should not be used for general use. High temperature paints have very little binder and will chalk (rub off) after ageing several months.
JDD August 22, 1987