US2224095A - Tube for heat exchanging apparatus - Google Patents

Description

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Vl R R A B M. F.

TUBE FOR HEAT EXCHANGING APPARATUS Filed Feb. l5. 1940 5 5 Y O D; Y, T W m o O. R 5 Poo mp Nw, Dm l L @am E Y1 YW W s u M TO. A H D L l/ FN HA Ao T m m2, E m m M. MT AFIRIM. D\ DN E ,W A Am m m m n m S L- Mlw I l D CBA m u u E E m M R n mlm \\\V\\F 5 \k\\w,\\ w MN 5 2 w O ma W m j i wm E TA MR s um m Tf @u um cH my mm DF Patented Dec. 3, 1940 UNITED STATES PATENT OFFICE TUBE FOR HEAT EXCHANGING APPARATUS Application February 15, 1940, Serial No. 319,158

Claims.

This invention or discovery relates to improvements in tubes for heat exchanging apparatus; and comprises, as an element in condenser systems or the like for use with aqueous fluids in which the solution pressures of copper and zinc are high, a `conduit or tube resistant to dezincincation and composed of an alloy like admiralty metal containing 0.001 to a maximum of 0.2 per cent phosphorus in uniform distribution through the metal, and the balance substantially all copper and zinc with a small proportion of tin and usually with a small proportion of modifying metals; said tube exhibiting a markedly lower solution pressure in contact with such fluids and l5 being capable of developing a protective nlm on the surface thereof in contact with moisture, protecting the tube from dezincincation; all as more fully hereinafter set forth and as claimed.

Copper-zinc alloys (brasses and bronzes) have 20 found extensive use in conduits for use in heat exchangers, etc., in installations Where the use of steel tubes is not feasible. Such alloy tubes give good service with many Waters, but in some l installations the phenomenon known as dezincin- 25 cation is encountered. A tube which has failed as a result of dezincincation exhibits a weak, porous structure, partly or entirely composed of copper, and often showing characteristic lamina.

Dezincincation of tubes is associated with Waters in which zinc, and to a less extent copper, has an abnormally high solution pressure. In ordinary water zinc exhibits a small solution pressure while that of copper is negligible. But in some aggressive Waters the solution pressure of copper and of zinc may be very high, relatively speaking.

Dezincincation is that form of brass corrosion which results in the formation of soluble copper and zinc compounds with redeposition of the dissolved copper at corroded areas, and resulting in a loss of zinc. The action may occur locally (plug type dezincincation) .or over broad surfaces (layer type dezincincation) As corrosion progresses 45 the tube wall is eventually transformed into a porous, spongy mass of copper, of characteristic appearance. Dezincincation is favored by (1) high operating temperature, (2) brackish, acid and other contaminated circulating waters, (3)

50 sluggish or stagnant waters, (4) certain types of permeable scale formations, (5) moisture-contaminated petroleum vapors especially at elevated temperatures. The higher zinc content alloys are the more susceptible of this type of cor- 55 rosion; those alloys containing les; than 15 per (Cl. 13S-47) cent zinc being essentially free under normal conditions of service.

The durability of brass tubes depends partly on the establishment and maintenance of an inert protective nlm in contact with Water, with of 5 course maintenance of massive non-corroded metal under and supporting the nlm. The constitution of the nlm depends on the character of the alloy and of the liquids to which the tube is exposed, and on other things. Often in con- 10 denser tubes the nlm is composed of oxides or hydrated oxides of both the alloyed metals, together with lime, etc., from the Water. Hard Waters are rarely aggressive to copper-zinc tubes, because of the scale formed thereby on the tubes. 15 Dezincincation is in part associated With a breakdown, or non-formation, of a suitable protective nlm. The same is true of other types of corrosion (concentration-cell action, galvanic type corrosion), etc. but a nlm which adequately protects brass condenser tubes from certain other types of corrosion may leave the brass Wide open to the dezincincation type of corrosion.

Dezincincation, an vend result not developed in other type of corrosion, is of outstanding importance in condenser tubes.

I have discovered that in copper-zinc alloy tubes the solution pressure of the copper and of the zinc can be eiectively restrained by the pres. ence of a minimal amount of phosphorus in solid solution therein. Dezincincation is inhibited. Tubes of copper-zinc alloys and in particular admiralty metal, which is an alloy of Z0 per cent copper, 29 per cent zinc and 1 per cent tin, can be markedly improved as regards resistance to 35 dezincincation by incorporation in the alloy of a small proportion of phosphorus! The proportion added is minute, ranging from 0.001 per cent to 0.2 per cent, and is uniformly distributed throughout the metal. A proportion of 0.03 per cent seems to give best results. Tubes made of this alloy in service in contact with moisture under conditions which would ordinarily produce dezincincation, develop a thin nlm containing phosphorus which effectively protects the body of the metal .against this type of corrosion. The eiect of the added phosphorus is out of; al1 proportions to the amount employed. Incidentally, the new alloys are characterized by annex' grain structure upon annealing which isirenected by an enhanced surface smoothness. 'Ihis is an additional factor in prolonging the life of the tubes.

In the tubes of the present invention, dezincincation is practically obviated though the general 'of no help in determining the dezinciflcation resistance of the brass. While any substantial amount of phosphorus is not regarded with favor in Ibrasses and bronzes, except in those intended for special purposes (such as springs, for example), I iind thatin the presence of a mere trace of phosphorus, insufficient to aect the physical properties of the brass materially, dezincification is substantially inhibited. A very small amount of phosphorus is suiicient to achieve this result. About 0.03 per cent is a good optimum. The proportion of phosphorus can be increased up to a maximum of about 0.2 per cent without substantial degradation of the physical properties of the alloy, but usually the desirable proportion of phosphorus is much less than 0.2 per cent.

The reason for the action ,of these minimal -additions of phosphorus is not k'nown, but the solution pressure of the alloy of copper and Zinc is effectively restrained. The ilm formed on the alloy tubes in service contains phosphorus.

The invention is especially useful for improving admiralty metal tubes but'is also useful with other copper-zinc alloys. In general, copper alloys containing more than 85 per cent copper exhibit little tendency to dezlncification and the addition of phosphorus is not so necessary. On the other hand, alloys with less than about 60 a 40 per cent copper find little use in the installations described. Muntz metal which iinds considerable use in condenser tubes,.='etc. is improved by phosuhorus additions as described and so are high copper alloys (85 per cent copper, 15 per centv4 zinc) and copper-zinc alloys containing other modifymade of the alloy of the invention to a standard ing elements than tin, such as aluminum brass which contains copper, zinc and a small amount of aluminum; often in proportions 76.Cu, 22 Zn and 2 Al. l

In preparing a condenser tube according to the invention, a suitable copper-zinc alloy melt is prepared and phosphorus in the'form of a copper phosphorus alloy is incorporated in the melt, or at any other suitable stage'of operations, in such proportion that the finished tube contains phosphorus in the speciiled proportions.

The phosphorus-containing alloy tubes in the ordinary way.

In the accompanying -drawing'there are presented charts illustrative of the improvement in dezincication resistance achieved in the invention. In the drawing` Fig. 1 isa diagram showing the eiect upon dezincication resistance. of exposure of articles is wrought into corrodingsolution, as measured by ductility,

Fig. 2 is a diagram corresponding to test data similar to Fig. 1 but in terms of tensile strength,4

and

Fig.l 3 is a perspective view of an `alloy tube according to the invention,

Referring to the drawing, Figs. 1 and 2 correspond to corrosion tests on specimens of the new alloys with various phosphorus contents and exhibit the resistance to dezincication achieved thereby. In making the tests, specimens of the several alloys were kept in an aggressive aqueous liquid, at a temperature of- F., for various lengths of time. For the sake of concreteness a 1 per cent copper chloride solution was employed as the aggressive liquid, Such a solution gives, at an accelerated rate, results quite comparable to those obtained with aggressive waters over longer periods of time. Dezinciflcation strictly speaking is measured by the loss of the zinc content of an alloy, but t'wo other parameters, ductility and tensile strength, are rather closely related to loss of zinc andA were used for the comparative tests. l

Fig. 'l shows, in curves A, B, C and D the effect on ductility of exposure for various lengths of time of test specimens of admiralty metal containing 70 parts copper, 29 parts zinc, 1 parttin, and phosphorus in the proportions, by weight on the alloy, of 0.01, 0.03, 0.0'7 and 0.12 per cent, respectively. Curve E shows the eiect for phosphorized Muntz metal (60 parts copper, 40 parts zinc and 0.01 part phosphorus). Curves F and G are for ordinary non-phosphorus-containing admiralty and Muntz metalsof' the prior art and are shown for the sake of comparison.

The curves show that the new alloys retain a usefully high ductility over prolonged exposure to the aggressive liquid. Thev ductility drops at first, probably due to corrosion, rather than def Muntz metals lose practically all their ductility Within the first few weeks.

Fig. 2 presents tensile strength data for the same set of alloys exposed to the same corrosive solution at the same temperature. The curves, a, b, c, d, e, f and g, correspond to curves A to G of Fig. 1 except that tensile strength is the quantity measured. The phosphorized alloys retain a high tensile strength (above 30,000 pounds per square inch)` indefinitely, while ordinary Muntz and admiralty metals very soon lose all useful tensile strength.

Fig. 3 shows a portion of-'a heat exchanger tub embodying the invention.

The flattening off of the curves for the phosphorus-containing alloys is indicative of the formation of a protective film. The lm varies with the character ofthe aggressive bath; but the relations indicated hold qualitatively for a wide variety of aggressive'liquids. Quite comparable results are obtained in actual condenser tubes exposed to a wide variety of aggressive fluids; moist oil refinery vapors, sewage, carbonate waters, etc. etc. f

This application is a continuation-impart of' my prior application, Serial No. 278,457, led June l0, 1939 for Copper base alloys.

What I claim is:

1. In a heat exchangingv conduit exposed to dezinciication-producingl conditions in contact.

Dezincification is effectively incel of copper substantially 76 parts per hundred, zinc substantially 22 parts per hundred and aluminum substantially 2 parts per hundred, means for inhibiting dezincifcation therein consisting of a quantum of phosphorus present in the finished alloy, in the proportion 0.001 to 0.2 per cent of the alloy.

' FREDERIC M. BARRY.

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