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Underground corrosion of galvanized steel
448
NATIONAL BUREAU OF STANDARDS NOTES
[J. F. I.
the float valve automatically closes. T h u s the pressure which, the mercury column exerts on the gas entering the capillary tube is kept constant, and the rate at which mercury flows out of the volume chamber is controlled by the rate of flow of gas through the capillary. In m a k i n g the determinations, the a p p a r a t u s is first pressurized at a specific temperature. Flow measurements are then carried out at decreasing pressure levels, and the viscosities are c o m p u t e d for each value of pressure. This procedure gives a family of isotherms showing the variation in the viscosity of the gas sample with pressure at the t e m p e r a t u r e of the test. In addition to its use in viscosity studies, the new flowmeter m a y find application in determining the specific gravity of a gas by the effusion method. For this purpose the capillary tube would be replaced by a thin metal diaphragm containing a small orifice. Because the viscosity of a gas changes rapidly with temperature, the Bureau's flowmeter should also prove useful for measuring temperatures, particularly at elevated levels where thermoelectric means have proved unreliable. For such use, the capillary tube would be coiled into a compact sensing unit which would be charged once and for all with a suitable gas. T h e time interval for a given volume of the gas to flow through the capillary would then be a measure of the average t e m p e r a t u r e of the capillary. UNDERGROUND CORROSION OF GALVANIZED STEEL
A s t u d y of underground corrosion of hot-dipped galvanized steel pipe has recently been completed by the National Bureau of Standards. Results confirm previous N B S work in showing t h a t galvanized steel having 3 ounces of zinc per square foot of exposed surface is highly resistant to corrosion in m a n y soils which are very corrosive to bare steel. C o n d u c t e d by Irving A. Denison and Melvin Romanoff of the Bureau's corrosion laboratory, the present s t u d y was an outgrowth of earlier N B S exposure tests of zinc coatings for underground use. For the Bureau's study, short lengths of both galvanized and uncoated steel pipe, and also plates of zinc, were buried at 15 test sites for periods up to 13 years. Located in widely separated parts of the United States, the test sites represented a wide range of soil properties. After each of five periods of exposure, a set of specimens of each material was removed and returned to the N B S laboratories. After removal of the corrosion products, determinations were made of loss in weight, d e p t h of the deepest pits, and the percentage of area of the galvanized specimens on which coating remained. Although the nominal weight of the zinc on all the coated specimens was 3 ounces per square foot, the actual thickness varied over a wide range. This was shown by a large n u m b e r of thickness measurements
May, I 9 5 3 . 1
N A T I O N A L BUREAU OF STANDARDS NOTES
449
made by a magnetic method, using unexposed samples of pipe from the same lot as the buried specimens. Additional determinations by an electrolytic stripping method showed that a large part of the zinc that was applied to the steel pipe was converted into alloys of zinc and iron. The zinc coatings provided good protection in most of the soils. In one soil in which bare steel pipe was perforated by corrosion after exposure for only a few years, the coating on the galvanized specimens remained perfectly continuous throughout the entire 13-year period. In only two of the 15 soils, both organic, was the zinc coating of negligible protective value. An interesting finding of the NBS study was the high corrosion resistance that the galvanized specimens continued to show in most of the soils after the outer zinc coating, and even after the zinc-iron alloy layer, had entirely corroded away. This continuing protection is tentatively attributed to an inorganic coating, probably silicious, believed to have been deposited by galvanic action between the outer zinc coating and the underlying steel or alloy layer. This tentative explanation is based in part on unpublished studies which indicate that the zinc-iron alloy layer does not protect steel sacrificially (cathodically), and that the alloy is no more resistant than steel to soil corrosion. Analysis of the data obtained in the course of the two NBS field studies shows that the minimum weight' of zinc coating required to protect steel for a minimum of 10 years depends on the nature of the soil environment. In the NBS studies a 2-ounce coating gave sufficient protection in inorganic oxidizing soils, but for inorganic moderately reducing soils a 3-ounce coating was required. Highly reducing soils, both organic and inorganic, require coatings heavier than 3 ounces per square foot. In order to obtain maximum life from galvanized pipe in practice, it is necessary either to construct the entire piping system of galvanized pipe or else to electrically insulate the galvanized sections from pipe of other metals. Otherwise, the zinc coating will be removed by galvanic action. NOTE: Further details of this NBS investigation are given in "Corrosion of Galvanized Steel in Soils," by Irving A. Denison and Melvin Romanoff, J. Research NBS, Vol. 49, p. 299 (Nov. 1952) (NBS Research Paper 2366; 10 cents from Superintendent of Documents, Government Printing office, Washington 25, D.C.) For information on earlier related NBS work, see "Corrosion of Metals Underground," NBS Tech. News Bull., Vol. 34, p. 48 (April 1950).
NATIONAL BUREAU OF STANDARDS NOTES
[J. F. I.
the float valve automatically closes. T h u s the pressure which, the mercury column exerts on the gas entering the capillary tube is kept constant, and the rate at which mercury flows out of the volume chamber is controlled by the rate of flow of gas through the capillary. In m a k i n g the determinations, the a p p a r a t u s is first pressurized at a specific temperature. Flow measurements are then carried out at decreasing pressure levels, and the viscosities are c o m p u t e d for each value of pressure. This procedure gives a family of isotherms showing the variation in the viscosity of the gas sample with pressure at the t e m p e r a t u r e of the test. In addition to its use in viscosity studies, the new flowmeter m a y find application in determining the specific gravity of a gas by the effusion method. For this purpose the capillary tube would be replaced by a thin metal diaphragm containing a small orifice. Because the viscosity of a gas changes rapidly with temperature, the Bureau's flowmeter should also prove useful for measuring temperatures, particularly at elevated levels where thermoelectric means have proved unreliable. For such use, the capillary tube would be coiled into a compact sensing unit which would be charged once and for all with a suitable gas. T h e time interval for a given volume of the gas to flow through the capillary would then be a measure of the average t e m p e r a t u r e of the capillary. UNDERGROUND CORROSION OF GALVANIZED STEEL
A s t u d y of underground corrosion of hot-dipped galvanized steel pipe has recently been completed by the National Bureau of Standards. Results confirm previous N B S work in showing t h a t galvanized steel having 3 ounces of zinc per square foot of exposed surface is highly resistant to corrosion in m a n y soils which are very corrosive to bare steel. C o n d u c t e d by Irving A. Denison and Melvin Romanoff of the Bureau's corrosion laboratory, the present s t u d y was an outgrowth of earlier N B S exposure tests of zinc coatings for underground use. For the Bureau's study, short lengths of both galvanized and uncoated steel pipe, and also plates of zinc, were buried at 15 test sites for periods up to 13 years. Located in widely separated parts of the United States, the test sites represented a wide range of soil properties. After each of five periods of exposure, a set of specimens of each material was removed and returned to the N B S laboratories. After removal of the corrosion products, determinations were made of loss in weight, d e p t h of the deepest pits, and the percentage of area of the galvanized specimens on which coating remained. Although the nominal weight of the zinc on all the coated specimens was 3 ounces per square foot, the actual thickness varied over a wide range. This was shown by a large n u m b e r of thickness measurements
May, I 9 5 3 . 1
N A T I O N A L BUREAU OF STANDARDS NOTES
449
made by a magnetic method, using unexposed samples of pipe from the same lot as the buried specimens. Additional determinations by an electrolytic stripping method showed that a large part of the zinc that was applied to the steel pipe was converted into alloys of zinc and iron. The zinc coatings provided good protection in most of the soils. In one soil in which bare steel pipe was perforated by corrosion after exposure for only a few years, the coating on the galvanized specimens remained perfectly continuous throughout the entire 13-year period. In only two of the 15 soils, both organic, was the zinc coating of negligible protective value. An interesting finding of the NBS study was the high corrosion resistance that the galvanized specimens continued to show in most of the soils after the outer zinc coating, and even after the zinc-iron alloy layer, had entirely corroded away. This continuing protection is tentatively attributed to an inorganic coating, probably silicious, believed to have been deposited by galvanic action between the outer zinc coating and the underlying steel or alloy layer. This tentative explanation is based in part on unpublished studies which indicate that the zinc-iron alloy layer does not protect steel sacrificially (cathodically), and that the alloy is no more resistant than steel to soil corrosion. Analysis of the data obtained in the course of the two NBS field studies shows that the minimum weight' of zinc coating required to protect steel for a minimum of 10 years depends on the nature of the soil environment. In the NBS studies a 2-ounce coating gave sufficient protection in inorganic oxidizing soils, but for inorganic moderately reducing soils a 3-ounce coating was required. Highly reducing soils, both organic and inorganic, require coatings heavier than 3 ounces per square foot. In order to obtain maximum life from galvanized pipe in practice, it is necessary either to construct the entire piping system of galvanized pipe or else to electrically insulate the galvanized sections from pipe of other metals. Otherwise, the zinc coating will be removed by galvanic action. NOTE: Further details of this NBS investigation are given in "Corrosion of Galvanized Steel in Soils," by Irving A. Denison and Melvin Romanoff, J. Research NBS, Vol. 49, p. 299 (Nov. 1952) (NBS Research Paper 2366; 10 cents from Superintendent of Documents, Government Printing office, Washington 25, D.C.) For information on earlier related NBS work, see "Corrosion of Metals Underground," NBS Tech. News Bull., Vol. 34, p. 48 (April 1950).