The Right Kind of Coating for Pipe

The Right Kind of Coating for Pipe

What Aim of Future Investigations Should Be—Research Should Be Made to Discover Composition That, Used with Tar, Would Seal the Pores of Iron or Steel Pipe

SPEAKING as an expert in matters connected with A the manufacture of steel and cast iron pipe, Mr.

Conard has made some excellent suggestions in the following article as regards the very troublesome question of a durable and efficient coating that will insure a

longer life for the pipe. This is a subject that is of vital interest to practically every water works superintendent:

When we study pipe for carrying water we naturally ask the question, why do we put a coating on this pipe? The answer is that we do not find it necessary in the case of wood, cement or terra cotta, but do in the case of iron or steel for the reason that they corrode, contaminating the water, and forming obstructions on the interior, thereby causing loss of volume and velocity of flow, and so deteriorating the walls of the pipe that eventually there is failure of the structure. The outside is also subject to deteriorating influences, particularly steel pipe, depending somewhat on the character of the soil and other influences.

William R. Conard Burlington, N. J.

Coating Used More than 70 Years Ago

When iron first began to be used for water pipe, I do not know what material if any was used to coat, them, but more than 70 years ago a process for coating cast iron pipe using coal tar was developed by Dr. Angus Smith and this process or variations of it have been in practically constant use since for cast iron, and it or coatings with an asphaltum base have been used for wrought iron and steel. For the smaller sizes of wrought pipe galvanizing has also been extensively used, as well as lead lined and cement lined. At the time Dr. Smith got out his process, “coal tar” was a residue of the manufacture of gas from coal, just as it is today, only in those days the many by-products to be obtained from this residue were apparently little known, and the tar was in every way and from every standpoint what is termed “virgin tar.” Could the good doctor have foreseen some things, such as by-products, tar from oil and water gas sets, etc., it is probable that he would have given much more detailed specifications, and placed many safeguards around the coating and process that we of the present generation may lose unless careful.

What Standard Specifications Provide For

The standard specifications of today say:

Every pipe and special casting shall be coated inside and out with coal-tar pitch varnish. The varnish shall be made from coal tar. To this material sufficient oil shall be added to make a smooth coating, tough and tenacious when cold, and not brittle nor with any tendency to scale off. Each casting shall be heated to a temperature of 300 degrees Fahr. immediately before it is dipped, and shall possess not less than this temperature at the time it is put in the vat. The ovens in which the pipe is heated shall be so arranged that all portions of the pipe shall be heated to an even temperature. Each casting shall remain in the bath at least five minutes.

The varnish shall be heated to a temperature of 300 degrees Fahr. (or less if the engineer shall so order), and shall be maintained at this temperature during the time the casting is immersed. Fresh pitch and oil shall be added when necessary to keep the mixture at the proper consistency, and the vat shall be emptied of its contents and refilled with fresh pitch when deemed necessary by the engineer. After being coated the pipe shall be carefully drained of the surplus varnish. Any pipe or special casting that is to be recoated shall first be thoroughly scraped and cleaned.

All of this follows somewhat, the specifications as first developed by Smith, but you will immediately note the term “coal tar pitch varnish.” Bitch and varnish are used a number of times when as a matter of fact the material is not pitch nor is it varnish ns they are generally understood. Bitch is the residue of tar after distilling, or is an extract of “pine tar,” and varnish is generally understood as being a resinous product. Another thing, the kind or kinds of oil and the proportions are not definitely stated, and in this connection variations in season and weather, and the opinions of individuals, should be offset by definite statement.

Composition of Average Coal Tar

Average coal tar is made up of carbons, oils, acids and gases. The acids and gases in the forms of hydrogen, oxygen, nitrogen, sulphur. Its main body is carbon and it is the carbon held together by the oils which protects the metal from corrosion, and it is the acids that tend to attack and corrode. Therefore it is desirable to get for pipe coatings a tar as free from acids as may be, and as high in carbon and oils, especially the heavier oils, and to use the coating at temperatures that will not evaporate the oils.

“While I do not say that tar should be discarded, research should be made to see whether there is something to use with the tar that will seal the pores, which will prevent the forming of the ferrous oxide and the formation of tubercles.”

Coal tar and the by-products that are being extracted therefrom are put to a wide variety of uses. The heavier material usually called tar (no matter how much it has been distilled or refined) is used for paving, road work, the varying forms of roofing materials, such as paints, etc., while the lighter [tart is used in many lines of industry particularly the chemical, so that its value is greater for refining purposes than in its crude state, which condition is the best for pipe coating purposes. The result is that unless the pipe maker controls some source of production, he has to compete with the refiner, with the consequence that there are times when he gets left, and the pipe purchaser is the innocent sufferer. This does not mean that the pipe maker is willing that this be so, but it is simply a stress of circumstances.

May be Necessary to Find Substitute

It may prove to be several generations in the future before the supply of bituminous coal from which we get out tar is exhausted, but as the by-products become greater in variety and value the keener the pipe maker will have to compete, and the greater will become the cost. At present the tar used for pipe coatings is about 15 per cent, of the total cost of coating pipe, and my thought is, when is t.he point going to be reached when economy ceases, in using coal tar, and what other material or materials can take its place?

It is not the purpose of this paper to attempt to technically cover the ground that the subject of pipe coatings brings up. and it is the belief of the writer that as regards “Present Day Tars for Pipe Coatings,” while the pipe maker specifies and tries to get “virgin tar,” that the tar does not contain as large a proportion of the heavy mineral oils as the tars of Dr. Angus Smith’s time contained, so that, there are cases where the coating is not as durable as it was once. It is my further belief that it is high time that careful and more extended studies, and research should be gone into to see what might be done about it, and I consider it entirely within the range of possibility that even using coal tars that have had their by-products extracted together with some other material or materials whether oils or some other form of hinder that a better and more durable coating than at present can be found. I would not say to discard tar entirely for, as l view it. there has not yet been found anything that could within economic limits take its place, but I do not think that the virgin tar of today is the last word in pipe coatings, and I do think thorough research would develop some other material to put with tar that would form a much better protection to iron and steel pipe surfaces than we are now using.

Some Research Work Already Done

When I speak of research work it does not mean that investigation has not been made, for it has. Dexter Brackett back in the Eighteen Nineties made tests, keeping Thomas Wiggin at the foundries for many months to try and get better coating; Allen Hazen in 1908-9 had some careful work done. Clarence Goldsmith, about 1913, for the Boston high pressure fire service in collaboration with Frank Mclnnes, tried a variety of tar coatings. The result of Mr. Brackett’s and Mr. Wiggin’s work in large measure, gives the specification as at present written. Mr. Goldsmith’s work gave a specification using tar and rosin, which gave a good appearing coating, but due to the temperatures used did not appear to have the lasting qualities required. The investigations of Mr. Hazen caused his conclusions to be for a tar distilled until the naphtha was removed with enough linseed oil to give a smooth, tough and tenaceous coating for steel pipe. As this apparently applies primarily to steel pipe, and I do not see it mentioned for cast iron it is rather assumed that the present standard requirements for cast iron are acquiesced in by Mr. Hazen.

Apparently all of these investigators looked up other materials, and by a greater or less eliminating process got back to coal tar. At the same time all apparently agree that the factors of: heat of the pipe for varying thicknesses, heat of the tar bath and keeping it of known proportions of fresh and old tar. knowing the temperatures of both pipe and bath and maintaining them at predetermined temperatures so as to obtain uniform results, are the prime factors, but all of this as indicated entails men of greater skill and intelligence than the average pipe coater; better, and more extensive apparatus and facilities for doing the work than exist as an average, even at the present time; and after these were attained a very strict adherence to them.

Aim of Future Research Work

As to the aim of research work which I say should be undertaken :

From what I have observed of pipe, which has been in service, and for some reason taken out, my conclusion is that the coating on the pipe is not impervious to moisture, having small pores which usually extend through the coating. This allows the moisture to get to the metal, ferrous oxide forms coining out through these pores, and spreading over the interior of the pipe. Perhaps you may have noticed in pipe taken out that when you take off some of the scales formed that you will find apparently good coating underneath. Therefore while as before stated I do not say that tar should be discarded, research should be made to see whether there is something to use with the tar that will seal the pores, which will prevent the forming of the ferrous oxide, and the formation of tubercles.

(Continued on page 1122)

Right Kind of Coating for Pipe

(Continued from page 1108)

You get somewhat the same effect when pipe is allowed to lay out in the weather except that in that case the ferrous oxide begins to form under the coating and causes the coating to scale off. Also I believe that investigation should be made of whether p pe of different sizes and classes should he dipped in the tar bath at different temperatures, for the volume of the metal in the pipe controls the number of heat units to he thrown off before the coating can become set, and as long as the heat prevents the setting of the coating, the heat will also continue to evaporate the oils in the tar and tend to leave nothing lnit dead pitch.

Some Suggestions as to Improvement

As to what materials might be used in conjunction with tar to prevent the forming of iron oxide, there are these things that have occurred to me. One is the coating especially of the interior of the pipe with some parrafin substance betore applying the tar; this was suggested casually by S. B. Brown, of Warren Foundry. The other is the use of some substance that has no oil or gas forming constituent, but is very dense and when set, impervious to moisture, which will become fluid at approximately the same temperature as tar, and will mix with tar readily, and can therefore be put in the tar tank, and applied in conjunction with the tar. And still another might he to find some means of getting a much thicker coating on the pipe, which would stay put, and not tend to scale in cold weather, and soften and run in hot weather. The city of Philadelphia did, and 1 think, still have a specification that the coating shall be at least 1/64 of an inch thick, which would help some but I do not think is thick enough, and with the existing methods in vogue at the foundries, and the tars used is quite difficult of attainment.

If simply tars are to be used every known constituent should be determined, and careful experiments made in an endeavor to determine which are beneficial and which harmful to pipe as a coating; just which will give the longest efficient life.


To summarize: Apparently present day virgin tars do not appear to contain as great an amount of the heavy oils as the old tars, and probably have more of the lighter oils which evaporate more readily.

Present day specifications are at variance with the process developed by Dr. Smith, and the demands of the pipe purchasers on the manufacturer have been such that the old Smith method, which was a rather lengthy operation and involved dipping the pipe cold in hot tar and allowing them to stay in the bath until the pipe became hot could not be worked today unless much more elaborate apparatus than that of Dr. Smith’s time were installed and this would be necessary to coat the present amount of pipe produced.

Research if undertaken should be along at least these two lines. The apparatus necessary to care for the present day volume of production, going back to a strict, adherence to the Dr. Angus Smith process or, using present apparatus and methods, or something similar in rate of working, and find something to mix with the cheaper distilled tars to get a coating that will be as lasting and protective or if possible more so than the old Smith process.

(Excerpts from paper read before the Philadelphia convention of the American Water Works Association.)

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