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STUDY AND REPORT ON PITTING AND CORROSION OF
M. E. McDONNELL, Chairman;
P. M. LABach,
An examination of technical literature discloses that discussion of this subject has been in progress for the last sixty years without any definite conclusion having been reached either as to the cause or prevention of pitting and corrosion.
In the meantime the power on railroads has increased tremendously both in first cost and in service required so that it has become of increased importance in the economical handling of traffic that suitable means be found for the prevention of pitting and corrosion of boilers with the incident losses that are involved.
It cannot be expected that this Committee can find the solution in a short space of time and this report is submited as a progress report only. The following is a brief resume of the chemical aspects in connection with pitting and corrosion :
The corrosion of steel and iron boiler tubes and sheets is believed to be the result of electro-chemical reactions. Iron, in contact with an electrolyte containing hydrogen ions, goes into solution as iron ions, thereby displacing hydrogen ions, which are liberated as molecular hydrogen. When water does not contain constituents which combine with the iron ions, a state of equilibrium soon results before the boiler tubes and sheets are perceptibly injured. When the water contains certain other constituents such as oxygen and carbon dioxide, the iron ions are removed as oxides or hydroxides, whereby the equilibrium of the solution is disturbed, and more iron goes into solution. A cycle is thereby established which may be rapidly destructive. The ultimate product of destruction is iron oxide, and it proceeds as fast as hydrogen is liberated. The rate of corrosion is a function of electromotive force and resistance of circuit. It is also a function of the active impurities in the water which combine with metallic ions, producing a magnetic oxide which forms a coating on the iron or steel surface which is entirely different physically from the voluminous hydrated precipitates resulting when oxygen or carbon dioxide is present. Free acids ionize in solution, yielding hydrogen ions and an anion radical or element which will combine with ionized iron in the absence of oxygen or carbon dioxide, thereby causing corrosion.
It follows that if the electromotive force could be reduced to zero, no corrosion would result, that if the difference of potential between two parts of a boiler were uniform, the corrosion would be uniform, and that local differences in potential may cause pitting or grooving. Fragments of mill scale adhering to or rolled into steel will set up a difference of potential at local spots, which when immersed in boiler water containing hydrogen ions and other elements capable of combining with diferrion, will cause pitting. Similar results follow the presence of other occlusions sometimes present in segregated steel, or in iron containing small inclusions of soft steel. Repeated alternate bending of a boiler sheet across one axis sets up a difference of potential along a corresponding line on the sheet. Corrosion starts along this line, and any protective coating, such as scale or magnetic oxide of iron, is kept broken by the bending operations, keeping clean metal exposed to the electrolyte. This eventually leads to grooving, unless the cycle of destruction is broken.
The following questionnaire was prepared at the Committee meeting in Chicago on May 6, 1921. This questionnaire was sent to all members of the Sub-Committee and replies were received from nine of the eleven members: (1) What In Your Experience Have Been the Causes of
Corrosion? The experience of the Committee indicates that general corrosion is prevalent where acid waters, or waters with a high percentage of chloride or sulphate hardness are used without treatment.
(2) What In Your Experience Has Been the Cause of Pitting? The replies indicate that pitting in locomotive boilers results from electrolytic action, the underlying causes of the electrolytic action being due to chemical or mechanical action or both.
The presence in the water of an electrolyte such as sodium sulphate or sodium chloride is necessary in sufficient quantities to convey the electric current between spots of impurities in the material which have different potentials, thereby resulting in a wasting of the metal at the negative pole. The presence of dissolved oxygen in the water materially assists the electrolytic action by removing the corrosion by-products and allowing the destructive action to continue..
(3) What In Your Experience Has Been the Cause of Grooving? The causes of grooving are also ascribed to electrolytic action. The strained metal of a boiler plate that is not completely covered by a rivet head becomes eaten away or perhaps a plate becomes strained at a point by temperature stress, and the strained streak is corroded. In each case the strained metal is of greater corrosibility and it acts as one of the plates of an electric battery, in which the other plate of the battery is the unstrained metal of the boiler shell and the electrolyte is the water in the boiler.
(4) What Is Your Method of Prevention? To date efforts toward prevention of corrosion, pitting and grooving have been chiefly confined to chemical treatment of the water. No means of complete prevention have as yet been found although in actual practice the trouble has been very materially decreased on some railroads by maintaining a caustic alkalinity in the boilers which appears to have an inhibitive effect on the corrosive action. Apparently, little work has been done toward improving the character of boiler materials and methods of handling with a view to securing a more homogeneous material and eliminating strains and stresses which appear to be responsible in part at least for much of the grooving, ring pitting and similar forms of corrosion. (5) Have You Laboratory Tests Confirming Specific Causes of 497
Corrosion, Pitting and Grooving? Laboratory tests were reported which indicated that pitting takes place in commercial boiler steel immersed in water very readily where oxygen and carbon dioxide are present and is accentuated by increased amounts of sodium sulphate or sodium chloride and also by higher difference of potential with an increased rate of electrolysis. It is also shown that this action is inhibited to a considerable extent by the addition of sodium carbonate or caustic soda. Other tests were presented which strongly indicated the presence of spots of impurities which had been rolled into the material and made up into flues.
It was brought out in Committee meeting that there is a marked difference in potential between material as originally furnished and that which had been subjected to strains and stresses during fabrication changing the crystalline structure of the metal. Attention was also called to advanced stages of corrosion of some material which had been affected by exposure to atmospheric conditions before being applied to boilers. (6) Have You Any Laboratory Tests to Suggest to Confirm
Your Findings or That Would Be of Assistance In the
Study of This Subject? Inasmuch as it has been found in practice, that treatment with caustic soda inhibits to a greater or lesser extent the pitting action, it is suggested that during the coming year careful tests be made to determine the minimum and maximum effective limit of caustic soda treatment with special relation to varying concentrations of sodium sulphate and sodium chloride, these being the principal electrolytes found in boiler waters.
This subject in general is intimately connected with departments other than engineering and it is respectfully suggested that the Board of Direction request that a committee be appointed from the mechanical section to work in coöperation with this committee, so that all phases including the manufacture and fabrication of material may be considered as well as the character of the water supply.
The photographs shown in Exhibits B and C are typical of conditions now occurring on railroads throughout the country.