The corrosion of iron; a summary of causes and preventive measures

Preface

This little volume is the result of an attempt to collect and put in simple form for reading and ready reference some of the more interesting and important facts connected with the corrosion of iron and its protection therefrom.

It requires no argument to prove the widespread nature and serious import of this process of decay, and the volume of literature to be found on this subject is evidence of the increasing attention which is being paid to it. It is only comparatively recently, however, that corrosion has been studied in a scientific manner and something of its true character ascertained, so it naturally follows that the same thing is true in large degree of the measures employed for protection. This is, perhaps, especially the case with paint pigments and vehicles, where careful experiments have brought to light many fundamental truths of the greatest value regarding the nature and action of some of these materials, thus enabling us to use them with much greater intelligence and effectiveness. Much of the published work, however, is scattered throughout many different scientific and technical magazines, consequently is largely inaccessible to one who does not have the time or facilities for such investigation.

It is accordingly hoped that this book, by presenting some of this material in condensed form, may help to give the student or busy engineer a better understanding of the problems involved in the successful preservation of one of our most useful building materials, and point the way to their solution.

Brooklyn, New York.

Chapter I. The Rust Problem

Its Importance to Constructing Engineers, Architects and Owners — Susceptibility of Iron and Steel to Corrosion — Economic Bearings of the Matter — What Corrosion Is — The Contest of Iron versus Steel — Effect of Modern High-speed Methods of Steel-making — Remarkable Examples of Preservation of Iron from Rust — Explanation of this Good Behavior of Certain Articles — The Chemical and Physical Facts Involved in Corrosion — Theory and Process of Solution — Ions and Ionization — Theory of Dissociation — Precipitation — Practical Bearing of these Things on the Rusting and Protection of Iron and Steel.

Chapter II. Theories op Corrosion ....... 23

The Three Principal Theories— The Hydrogen Peroxide Theory — Evidence for and against It — The Carbonic Acid Theory — Experiments Showing Its Partial Truth — The Electrolytic Theory — Data Supporting It — How It Agrees with Observed Facts — How It Influences the Choice of Protective Measures — Bad Influence of Stresses and Strains — Of Pickling — Of Indentations and Roughness of Surface — Conclusions.

iii

IV CON'TEN'TS

Chaptee III. Protective Measuees 46

Practical Application of the Principles Discovered — Three Methods of Protection: by Coatings of Oxide, by Coatings of Other Metals, by Covering with Paint or Similar Materials — Pioneer Work in Oxide Coatings by Lavoisier and Faraday — The Bower- Barff Process — The Wells Process — The Gesner Process — The Dewees-Wood Process — The Bontempi Process — The Buflfington Process — Heat Bluing — Coslettizing — Protection by Coatings of Other Metals: Galvanizing by Dipping, Electrolysis, and Vapor — Hot Galvanizing — Methods of Testing — Electro-depositing of Zinc : Methods and Defects — Tapor Processes — Sherardizing: Details and Advantages of the Process — The Molten Zinc Process — Preece and Walker Tests for Galvanized Work — Copper-coating — Nickel-coating — Leadcoating — Lohmannizing — The Schoop Process.

Chapter IY. Paint Materials 74

Wide Applicability of Painting as a Protective Measure — Xatnre of Paint — Two Methods of Making Paint — Comparative Merits of these Different Procedures — Ingredients of Paint — Linseed Oil — Drying and Driers and Their Effects on the Quality of the Oil — Adulterants of Linseed Oil — Other Paint Oils — Tung or China Wood Oil — Soya Bean Oil — Menhaden Oil — Thinners — Turpentine — Petroleum Oils — Pig-

CONTENTS V

ments — Their Influence on Durability — Findings of the Committees of the American Society for Testing Materials Tabulated and Discussed — Pigments Described — Zinc Chromate — Zinc and Barium Chromate — Zinc Oxide — Zinc-lead White — Barium Chromate — Ultramarine Blue — Chrome Green — Prussian Blue — Lithopone — Willow Charcoal — Litharge — Basic Carbonate-White Lead — Calcium Sulphate — Princess Metallic Brown — Orange Mineral — Calcium Carbonate — Sublimed Blue Lead — Lemon Chrome Yellow — ' Orange Chrome Yellow — Medium Chrome Yellow — Chrome Green — Venetian Eed — Bone Black — Asbestine — China Clay — Eed Lead — Indian Eed — American Vermilion — Sublimed White Lead — Mineral Black — Barytes — Graphite — Ochre — Carbon Black — ■ Lampblack.

Chapter V. Protective Paints 100

Quality of Paint for Ironwork Depends on Inhibitive, Insulating and Excluding Properties — Action of Various Pigments Discussed — Tests of Paint Films — Cushman and Gardner's Eesults — Tests Made by the American Society for Testing Materials — Formulae of the Paints Tested — Eesults — Conditions Controlling the Character of a Paint Film — How to Work up a Good Paint Formula — Effect of Various Ingredients — Japans and Enamels — Lacquers — Preparatory Treatment of Iron and Steel Surfaces before Painting.

VI CONTENTS

Chapter VI. Influence of Different Ele^

MJENTS ON the CoRROSION OF IrON 127

Definite Work of Eecent Date — Manufacture of Iron Summarized to Show Nature of the Problem — Natural Impurities and Their Effect on Electrolysis — Influence of Normal Constituents Individually Determined — Silicon — Sulphur — Manganese — Arsenic — Cobalt and Nickel — Silver, Tin, Tungsten and Selenium — Copper — Tests of Copper- Steels — Buck's Eesults Tabulated — Excellent Eesults Secured in Protection from Corrosion under Severe Conditions — Hopeful Outlook for Copper-Bearing Steel as a Rust Eesister.

Chapter VII. Corrosion of Wrought-Iron

AND Steel Pipe 149

Controversy over their Eelative Merits — Importance Due to the Enormous Dse of Pipe — Puddling and Bessemer Processes Summarized — Their Tendencies Pointed Out — ^Wr ought Iron Subject to Lamination — Bessemer Steel Free from this Tendency — Tests Made by the Navy Department — Eesults — Data Collected by the American Society for Testing Materials — Thomson's Experiments — Tests of the American Society of Heating and Ventilating Engineers — Woolson's Tests in New York City Public Baths — Walker's Experiments on Pipe Lines — All Conclusions Negative to the Superiority of Iron over Steel as to Corrodibility, and Tendency to Pitting Somewhat Greater in the Case of Wrought Iron.

THE CORROSION OF IRON

A MONG the many questions which the ar- -^ ^ chitect and builder are called upon to consider, none perhaps, is more important than that relating to the corrosion of iron and steel. This tendency to decay is peculiar to these materials, since none of the ordinary metals exhibits it to any comparable degree, and it is so strong that an unprotected piece is soon reduced to a shapeless mass of rust. It is one thing to design and erect a mighty sky-scraper, but an entirely different matter to protect it from those influences which, if allowed to do their work, would ruin the greatest structure in a short time.

It seems strange, in a way, that unless iron is well protected it is far less resistant to natural agencies than wood or other building materials. It is being used more and more in the fabrication of lar^e buildincrs and other

2 COEEOSIOIT OF IRON"

structures of a permanent type, and the length of their life, to say nothing of the safety of the people employed in and around them, depends on the success with which corrosion can be prevented.

Painters are constantly employed on such structures as the Brooklyn or Forth bridges, scraping away rust spots and repainting. When serious rusting is once started it is very hard to check, and as it would inevitably lead to the destruction of the framework, a collapse would be certain to occur sooner or later, probably with an appalling loss of life.

The economic side of the question is also of serious import. It is being recognized more and more that the increasing use of materials is depleting our stock of natural resources, and it is evident that when these are irrevocably lost through some form of decay, we have sustained a real and serious loss. The production of pig iron in the United States alone has increased amazingly until now many millions of tons are produced annually. How much of this is wasted for lack of suitable protection, or otherwise, cannot be stated very accurately, but the total is considerable. How long our ore supply will hold out against the steadily increasing demand can only be vaguely guessed, but if present conditions keep up there is certain to come a time when the scarcity and high price of iron will not permit it to be used so prodigally as now. Again, the coal supply is by no means unlimited, and since it requires about four tons of coal, or its equivalent, to produce one ton of steel, it is apparent that when we allow iron to rust or otherwise go to waste we are losing not only iron, but coal, one of our most valuable assets.

I do not mean to imply that the corrosion of iron is a new problem, or that technologists generally do not realize its importance, since the articles and discussions which have been appearing in the technical press for the last few years are evidence of the work that is being done along these lines. This literature, however, is distributed through a large number of publications and is, therefore, not readily accessible to the busy engineer, so in this small volume an attempt will be made to collect some of this information regarding the study of corrosion and its practical application in the protection of iron and steel, and to present it, together with the results of personal observations and tests, in a simple, compact form.

4 COEEOSION" OF lEON

Before taking up any of the theories as to the nature and cause of corrosion, it may be interesting to note a few points about the process in general. It is a matter of common knowledge that iron soon becomes covered with a heavy coating of rust, if it is exposed to moisture without first being protected by paint or other good preservative, and that in time the piece will be entirely honeycombed and will assume a lace-like appearance. It is also known that different pieces of iron and steel exhibit widely varying tendencies in this respect ; that is, some are surprisingly resistant and keep in fairly good shape for a long time even under very adverse conditions, while others soon show the most serious rusting when exposed to the same conditions. The manufacturers of wrought and charcoal irons and the various kinds of steel have each contended strenuously that his particular product is most resistant to corrosion, but it is not the province of this chapter to state who is in the right, since it appears that each of these types of metal may vary greatly in rust resistance and that there are good and bad irons as well as good and bad steels.

There is plenty of good evidence to support this view, and it seems worse than useless, therefore, for mill men to engage in wordy controversy over tlie particular merits of their products. The recent researches of several investigators show that internal stresses or strains in a metal, as well as improper treatment resulting in occluded gas and a porous metal with blow holes, are conducive to rapid corrosion. Modern high-speed methods, unless carefully performed, tend to produce a metal in which various substances, which are present either as impurities or are added to produce certain effects, are more or less segregated instead of being thoroughly mixed, and the iron is pounded and rolled into shape and cooled so quickly that the molecules do not have time to adjust themselves and heavy internal strains may be produced. Of course, the remedy for all this is more careful workmanship and adequate annealing. Metal which is strained beyond its elastic limit can be shown to be more easily corroded than metal which has not been strained, and it apparently makes little difference whether these strains are produced during the process of making the part or afterward.

There seems to be no doubt that at least some of the metal produced previous to the b COEROSIOX OF IROX

introduction of modern metliods (modem as far as speed is concerned) was superior in its resistance to corrosion to tlie present-day product, although the latter should not be condemned too hastily on this one point alone. Thus, I have seen various iron articles, especially nails, which showed far less rust after an exposure of a hundred years or so, than the modern variety sometimes does in a few weeks.

Another piece, an old flint-lock pistol, was especially interesting. It was found by a friend in a patch of woods in Vermont and had evidently lain there for many years, since a piece of newspaper dated 1796 had been used as wadding in loading it. All the iron parts were rather rough and pitted and covered with rust, but the arm was in surprisingly good shape, considering the conditions to which it had been subjected. The spring, hammer, and trigger were still capable of performing their functions and very little effort was required to put the old weapon in decidedly presentable condition after its extraordinary exposure to the weather.

The old bridge at Newburyport, Mass., has also attracted much attention because after something like a hundred years' exposure to the elements it shows remarkably little evidence of deterioration.

Going farther back, a number of iron articles have come down to us from ancient times, and perhaps the most noted of these is the famous pillar in the temple of Kutab Minar at Delhi, India. This old shaft, which projects some 30 feet above the surface of the ground, was erected about 900 B. C. Today it shows little trace of rust, although it has had no protective coating other than that which the process of manufacture and the atmosphere itself have formed upon it.

In this connection mention may also be made of what are known as the Ceylon link and the Singhalese chisel, nail and bill hook, all specimens of great antiquity.

To explain how or why these various articles have so successfully withstood the attacks of air and moisture for so many years is not as easy as it might seem, for apparently some of our commonly accepted notions and theories as to what a good iron should and should not be do not hold, or at least are offset by other conditions. For instance, some of the links of the Newburyport bridge have been examined and found to be of very ordinary purity and of a most heterogeneous

8 CORKOSIOIT OF IRON

structure, segregation of the impurities being decidedly noticeable. The question of any inherent superiority in the iron itself was settled by heating and rolling one of the links into a sheet and exposing it to the weather, when it was found that it corroded quite as readily and extensively as the material of today.

Again, tests made on the old relics referred to above show that only in manganese content do they approach modern irons in composition; the phosphorus is from 22 to 60 times and the silicon from 9 to 50 times as high in these as in the present product. From this it would seem that on the score of composition they are in no way superior to our own material, and in addition they contain several per cent of slag and cinder, which are admittedly very undesirable.

The explanation of their good behavior in service is probably found in the fact that the iron from which they were produced was refined, and the objects themselves fashioned slowly and laboriously by hand in a sort of forge. The prolonged heating and slow forging would tend to insure thorough annealing and freedom from strains, while the constant hammering produced a dense skin on the sur-

THE BUST PROBLEM 9

face, and this was doubtless covered with a more or less heavy coating of scale or oxide which also helped to protect the metal beneath. Thus it seems that the ability of these irons to withstand atmospheric attack is incidental to the method of working, rather than due to greater purity or freedom from segregation, as compared with modern irons. Of course, to apply such methods to the enormous output of iron and steel nowadays would be utterly impossible.

Mention has been made of the impurities in iron, and it may not be out of place to enumerate some of them at this time. All except very highly purified iron contains silicon, sulphur, phosphorus, manganese, and combined carbon, at least in small amounts. In making alloys, chromium, vanadium, tungsten, nickel, copper and several other elements may be added. Iron alloys or combines with practically all the other elements, and is so easily influenced by some of them that oftentimes only a very small percentage is required to alter its character markedly; as a consequence, specification requirements are generally drawn very rigidly.

A good deal of importance has been attached to the influence which sulphur, phos-

10 . COREOSION" OF IROK

phorus, and the other normal constituents of iron are supposed to exert on the process of corrosion, and manganese, in particular, has come in for an unusual amount of condemnation. For example, it is known that it decreases the electrical conductivity of iron, to a certain point ; therefore, unless it is uniformly distributed, the electrical conductivity will vary in different parts and on different surfaces, which has an important bearing on the problem in hand, as will be seen later on. Further, it is well known that manganese and sulphur tend to combine when both are present in iron and steel, and the resulting sulphide shows a difference of electrical potential against iron, which is a highly undesirable condition. In general, it may be said that, according to the electrolytic theory, any condition which tends to produce a difference of potential between surrounding parts is bad and will excite or accelerate rapid corrosion.

A detailed discussion of the influence of the various elements, as far as corrosion is concerned, will be taken up later on, but it may be said here that the results of a vast amount of experimental work show that even extremely pure iron corrodes about as readily as a good, commercial grade of material; therefore the question is apparently not so much what amount of impurities may be present as whether these are thoroughly distributed throughout the metal or segregated here and there in patches.

Before we can discuss some of the theories regarding the formation of rust and thus approach the subject in a scientific manner, however, it will be necessary to review briefly certain chemical and physical facts involved in the process, in order that there may be no misunderstanding of the terms and principles employed. Primarily we will be concerned with the phenomenon of solution.

Water is the great solvent for all forms of matter and there is probably nothing which it does not affect in some way. Most chemical changes take place in water, or in its presence, so it is not only destructive in its action, as we often feel, but also constructive, as new compounds may be formed as well as old ones broken up. When we speak of the solubility of a substance we refer to the ease with which it goes into solution; therefore we say it is insoluble if we find that no appreciable amount has been dissolved. Practically, however, there is almost nothing which is not dissolved to some extent, so the term is only relative, and there are all degrees of solubility ranging from those cases in which the solvent dissolves but an infinitesimal amount of the material, to those in which it takes more than its own weight of the substance into solution.

This is not the place to discuss why a certain solid does or does not dissolve in a certain liquid, so we will confine ourselves to substances which are easily recognized as soluble in water, that is, dissolve readily in it. When such a body is placed in water it disappears from view more or less rapidly, and its molecules, if it is a compound body, distribute themselves uniformly among the molecules of the solvent. This distribution is produced by a very definite force, known as the ** solution pressure,'' which is exactly analogous to the pressure exerted by a gas confined in a closed vessel. Under such circumstances the gas tends to occupy all the space at its disposal, and therefore produces a certain uniform pressure on all parts of the vessel. If we had steam at a pressure of 100 pounds per square inch in a boiler and wished to introduce more steam into the latter, it is evident that we would have to have the incoming vapor at a pressure slightly higher than 100 pounds per square inch, in order to overcome the back pressure, as it is called. Again, as a boiler is generating steam and the tension increases, it becomes ever harder for the water to assume the vapor state, against the pressure of the steam already formed.

The same thing happens in the case of solutions. As more and more of the substance dissolves, there is produced a back pressure, known as osmotic pressure. It is evident that when the osmotic and solution pressures are equal, no more of the material will go into solution and a state of equilibrium will result. If any outside influences disturb this equilibrium and lower the osmotic pressure, for example, it is easily seen that more of the substance will pass into solution until equilibrium is again established. Osmotic pressure may be of surprising magnitude, in certain cases, and can be shown by placing a glass tube closed at one end by animal parchment and filled with alcohol, in a glass of water. Alcohol and water have a strong tendency to diffuse or mix, and water can easily pass through the parchment into the tube, but the alcohol cannot; therefore the column of liquid in the tube will rise, showing an increased pressure. It may be interesting to note that the solution pressure of a very soluble substance, like sugar, is considerable. One investigator found that the pressure in the case of a 6 per cent sugar solution amounted to nearly 60 pounds per square inch.

Coming now to the behavior of substances in solution, we know that if we dissolve salt in water it apparently disappears, but since we realize it must be present in some form we conclude that it has been resolved into extremely fine particles. Experiment has shown that such a solution will conduct a current of electricity and two entirely new substances will make their appearance, one around each electrode, so it is plain that something in the solution is capable of carrying the electricity from one pole to the other, and that in so doing certain changes have taken place. The term *4on" was introduced, therefore, to denote the particle of matter which travels in the solution, along with the electricity, toward the poles. A further distinction is made in that the particle of matter which travels toward the anode or positive pole is called an an-ion, and that which migrates toward the cathode or negative pole, a cation.

The theory of dissociation holds, at the present time, that when acids, bases, and salts are dissolved in water they break down into ions, which are considered to be atoms or groups of atoms carrying large charges of static electricity. These charges are not apparent in the undissociated molecule, because the positive charges exactly neutralize the negative ones. Thus, hydrochloric acid dissociates into positive hydrogen and negative chlorine ions; sodium hydroxide (caustic soda) forms positive sodium and negative hydroxyl ions; ferrous chloride forms positive ferrous and negative chlorine ions, and soon.

Putting it in the form of a principle, without taking the time to demonstrate it, we may say that the cation of all acids is hydrogen; the anion varies with the acid. Likewise, the anion of all bases is the hydroxyl group (OH) ; the cation varies with the base, the metal with which it is combined. Since hydrogen is the characteristic ion of all acids, it can be seen that whenever its ions are present, acid properties are exhibited; in like manner, hydroxyl is the characteristic ion of

16 COREOSION" OF lEON"

all bases, and wlien liydroxyl ions are present basic properties are in evidence. With salts, both the anion and cation vary according to the elements composing the salt. It is not possible to discuss this interesting topic further, but anyone who wishes more information along these lines should consult some good text book on physical chemistry. Perhaps the application of the foregoing to the corrosion of iron may seem a little obscure, so we shall anticipate enough to say that this process is greatly retarded or accelerated by the presence of different ions, therefore some knowledge of their formation and action is necessary.

Another form of breaking down or dissociation, known as hydrolysis, is also of importance in the study of corrosion. If a solution of the carbonate, say, of sodium, potassium, or other strongly basic metal be tested with litmus paper, it will be found to have a strongly alkaline reaction. On the other hand, chromium, bismuth, aluminum and other weak bases tend to form salts which have an acid reaction; some are even too weak to exist. The principle may be deduced, therefore, that a compound formed from a weak acid and a strong base will be alkaline in reaction, while one formed from a strong acid and a weak base will be acid.

Wben considering the theory of dissociation it was said that hydrogen ions are the cause of an acid reaction and hydroxyl ions the cause of an alkaline reaction. Conversely, it can be said that an acid reaction denotes the presence of hydrogen ions, while hydroxyl ions are shown by an alkaline reaction. It has just been noted that different ions may exert great influence on the process of corrosion ; therefore, when we go into this more deeply later on we should be able to explain, in many cases at least, why certain pigments are stimulators of corrosion, or why they tend to act on the oil in paint films and cause early decay and failure. Basic carbonate of lead — white lead — for example, is so alkaline in its reaction that it saponifies or makes lead soaps of the linseed oil, leading to what is called chalking and deterioration of the paint film.

A further investigation of solutions shows that some will conduct a current of electricity while others will not. A substance which will conduct a current when in solution is called an electrolyte. For example, potassium nitrate, saltpetre, in solution will conduct a current very readily, but syrup offers a high resistance to its passage. It can be shown that potassium nitrate ionizes highly in solution, which is not true of sugar, and it has been noted previously that when a current is passed through a conducting solution a mechanical movement of the ions takes place toward the electrodes, where they give up their charges and ^ Opiate out'' in metallic or gaseous form. This may be familiar to the reader as the process of electro-plating. If a solution of zinc sulphate is made to conduct a current, the positive zinc ions proceed to the negative pole, give up their charges, and assume the atomic condition. An equivalent amount of sulphuric acid is set free at the same time at the positive pole. If this is of zinc, as in electro-galvanizing, the acid attacks it and a further quantity of metal is taken into solution. Going a step farther, it is not necessary that an external source of electricity be provided before electrolysis can take place. Strips of two different metals will generate a current when they are put into an electrolyte and the upper ends touched together or joined by wire. This may be explained by saying that the more electro-positive metal shoots positive ions into the solu-

THE EUST PROBLEM 19

tion and thus leaves itself negatively charged, consequently appearing as the negative pole. It is more or less rapidly disintegrated, while the other is protected. As will be shown later on, this phenomenon plays a very important part in the preservation of iron, especially by zinc coatings.

As everyone knows, all but a few of the elements are capable of combining and forming compounds with the other elements. The capacity of an atom to combine with other atoms is known as its '^valence'' and varies with the different elements. For instance, one atom of chlorine can combine with only one atom of another element; one atom of zinc can combine with two monovalent atoms, and so on. Sometimes the same element varies in its ability to combine with others ; for example, copper combines sometimes with one and sometimes with two atoms of chlorine. Manganese, gold, iron, and many others exhibit this peculiarity, which is thought to be due to the electrical state of the ion. By appropriate means such an element may be made to combine with a larger quantity of another element, if it is in the lower state of oxidation, or to give up a part of the element with which it is combined, if it is in the higher

20 SOEROSiON OF IRON"

state. Passing from a higher to a lower state of valence is known as '* reduction" and from a lower to a higher state as ** oxidation." Often one of these states is very unstable, tending to pass into the other under ordinarycircumstances. Thus iron forms two classes of compounds, ferrous and ferric, which have a valence of two and three, respectively, but the ferrous compounds oxidize very easily to the ferric state.