GB2056486A - Corrosion Resistant Copper Base Alloys for Heat Exchanger Tube - Google Patents

Abstract

The single phase alloy essentially consists of from 3 to 7.5% by weight nickel, 1 to 4% by weight aluminum, up to 3% by weight iron, 0.001 to 1.0% by weight manganese, and the balance substantially copper, but there may be optionally present up to 1% Cr, up to 3% Co (with the proviso that Co+Fe NOTGREATER 4) and 0.0-2% each of As, Sb and P. The alloy may be homogenised at 500-1050 DEG C, hot worked to finish at NOTLESS 400 DEG C, and cold rolled, if necessary with intermediate anneals at 50 DEG -1050 DEG C. Following hot working and annealing the alloy is water quenched to retain a solid solution.

Description

SPECIFICATION Corrosion Resistant Copper Base Alloys for Heat Exchanger Tube Copper base alloys have been extensively utilized in tubing for heat exchanger applications. These alloys, in particular the copper-nickel alloys, have been utilized for their good balance of corrosion resistance and fairly low cost. In particular, such alloys as Alloy 706 and 715 (containing, respectively, 10% and 30% nickel in a copper base) have found wide acceptance in surface condenser heat exchangers utilized by power generating plants. These alloys, although widely used, do present difficulties of their own. In particular, at least 10% nickel is usually necessary in the alloys to achieve good corrosion resistance. This tends to make the alloys quite expensive and therefore uncompetitive with other alloy systems.The initial corrosion rates for these copper-nickel alloys also tend to be quite high until a protective film has had a chance to form on the tubing surface formed from such alloys.

This high initial corrosion rate raises the possibility of copper being released to the environment and in particular to potable water flowing through tubes made from such alloys. The presence of large amounts of ionic copper in industrial effluents has been shown to be harmful to some aquatic species.

Therefore, research has been done into various alloy systems to determine an alloy which reduces or eliminates such copper release without being overly expensive.

Various alloy systems and particularly the aluminum bronzes have been developed to overcome the disadvantages of the copper-nickel alloy systems. These alloy systems generally have not been able to provide the high corrosion resistance properties of the copper-nickel systems in heat exchanger applications. Various alloy systems have been developed for their corrosion resistance properties which utilize many and varied alloy additions for such properties. For example, the Mikawa Patents 3,416,915 and 3,901,692 have taught copper base alloys containing various percentages of aluminum, nickel, boron, iron, silicon, titanium and carbon. These alloys have generally been described as being quite useful for producing corrosion resistant springs.These particular patents do not, however, discuss any possible use of these particular alloy systems for tubing in either heat exchanger applications or for potable water.

It would be desirable to provide an alloy system which is highly resistant to corrosion without being high in cost.

The alloy system as particularly disclosed herein provides increased resistance to corrosion in potable and brackish water applications compared to commercially available corrosion resistant alloys; exhibits a low initial corrosion rate to avoid soluble copper release to the environment on start up of tubing systems; and moreover retains single phase properties within the alloy structure after processing to increase corrosion resistance properties.

The alloy system of the present invention utilizes alloying additions of nickel, aluminum, iron and manganese in a copper base with optional additions of chromium and cobalt. Further alloying elements such as arsenic, antimony and phosphorus may be included in the alloy system as inhibiting agents.

This alloy system exhibits improved corrosion resistance in potable and brackish water conditions when compared to the widely used Alloy 706 (copper-10% nickel). This alloy system should be processed in such a manner as to maintain a single phase within the alloy structure since multiple phases within the structure have an inherently detrimental effect upon corrosion resistance performance.

In the accompanying drawings:- Figure 1 is a graph comparing the weight loss in potable water performance of several versions of the alloy system of the present invention when compared to Alloy 706.

Figure 2 is a graph comparing the weight loss in synthetic brackish water of several versions of the alloy system of the present invention when compared to Alloy 706.

Figure 3 is a graph comparing the weight loss in tap water of an alloy system of the present invention when compared to Alloy 706.

Figure 4 is a graph comparing the weight loss in synthetic brackish water of a specific version of the alloy system of the present invention when compared to Alloy 706.

The alloy system of the present invention incorporates the addition of various alloying elements in a copper base. In particular, these elements are 3.0 to 7.5% by weight nickel and preferably 3.0 to 5.4% by weight nickel, 1.0 to 4.0% by weight aluminum, up to 3.0% by weight iron and 0.001 to 1.0% by weight manganese. Up to 1.0% by weight chromium and up to 3.0% by weight cobalt may also be added to this alloy system, provided that the maximum iron plus cobalt addition is 4.0% by weight.

From 0.01 to 2.0% by weight of an element selected from the group consisting of arsenic, antimony and phosphorus, or combinations thereof, may be added to the alloy system as parting inhibitors.

Preferably, the alloy system of the present invention consists essentially of 4.0 to 6.0% by weight nickel, 2.0 to 3.0% by weight aluminum, 1.0 to by weight iron, 0.5 to 0.3% by weight chromium, 1.0 to 2.0% by weight cobalt, 0.1 to 0.5% by weight manganese, balance copper. The preferred range for the iron plus cobalt addition should be from 1.0 to 3.0% by weight. The elements listed above as parting inhibitors may also be added, singly or in combination, to the preferred alloy system.

The processing of this alloy system follows conventional practice, provided that the alloy retain its single phase throughout ail steps of the processing. The alloy system undergoes both hot and cold working to an initial reduction gauge, followed by annealing and cold working in cycles down to the final desired gauge.

The alloy of the present invention may be cast in any convenient manner such as Durville, direct chill or continuous casting. The alloy should be homogenized at a minimum temperature of 5000C and a maximum temperature of 1 0500C, or the solidus temperature, whichever is lower for the particular alloy, for at least 1 5 minutes. This homogenization is then followed by hot working of the alloy, for example by hot rolling, at a finishing temperature of at least 4000C and preferably between 650 and 9500C. The alloy should be rapidly quenched, preferably using a water bath, after being hot worked in order to insure a solid solution microstructure within the alloy.

The alloy is then cold worked at a temperature below 2000C with or without intermediate annealing depending upon the particular gauge requirements in the final strip material. In general, annealing may be performed using either strip or batch processing with holding times of from 10 seconds to 24 hours at temperatures ranging from 5250C to 1 0500C or within 500C of the solidus temperature for the particular alloy. Following annealing, the alloy is rapidly quenched, preferably using a water bath, to retain a single phase microstructure.

The process of the present invention and the advantages obtained thereby may be more readily understood from a consideration of the following illustrative examples. All percentages for the alloying additions will be in terms of weight percent.

Example I An alloy containing 5.21% Ni, 2.05% Al, 1.08% Fe, 0.16% Mn, balance Cu was cast as a Durville ingot and was hot and cold worked by conventional practice to a 0.12" gauge. The worked material was then annealed at 8500C for 1 hour, cold worked to 0.060" gauge, final solution annealed at 850"C for 1 5 minutes and finally cold worked to a 0.030" gauge. A sample from this material was tested along with Alloy 706 (both as strip material) for 90 days in New Haven, Connecticut potable water, which is an aggressive soft water known to be corrosive to copper base alloys. The strips were placed in a trough through which the water flowed at 3 feet per second (fps).The temperature of the water was controlled at 400C and the water supply was replenished at the rate of 10% per hour, thus simulating a once-through flow system. The Alloy 706 strip was commercially produced. Weight loss in milligrams per square centimeter of strip material was plotted against time in days for each alloy and the results are shown in Figure 1.

A second test was run in a similar trough containing 0.1% by concentration synthetic sea water formulated from ASTM Standard Specification Dl 141-52. This solution was recirculated but not replenished. Although the material was not replenished during the testing, the solution was changed weekly throughout the duration of the test. This simulated brackish water conditions. The weight loss for each sample in milligrams per square centimeter was plotted against time in days and the results are shown in Figure 2. As can be seen from both Figures 1 and 2, the alloy of the present invention has much lower initial and steady state corrosion rates compared with Alloy 706 in potable water.The alloy of the present invention exhibits slightly worse corrosion performance than Alloy 706 in brackish water although corrosion performance is still acceptable for commercial applications. The corrosion rates and localized corrosion observations for each sample are shown in Table I located at the end of these examples.

Example II An alloy containing 4.88% Ni, 2.02% Al, 1.04% Fe, 0.18% Cr, O. 1 7% Mn, balance Cu, was processed in a similar manner as in Example I, with the exception that the annealing temperature was 9000C. This alloy was tested in a similar manner as noted in Example I against Alloy 706. The weight loss results are shown in Figures 1 and 2 and both the corrosion rates and localized corrosion observations are shown in Table I.

It can be seen from Figures 1 and 2 that this particular alloy of the present invention has much lower initial and steady state corrosion rates compared with Alloy 706 in potable water, together with freedom from localized corrosion. As was the case with the alloy of Example I, corrosion performance in brackish water compared to Alloy 706 is somewhat worse but still acceptable for commercial applications.

Example Ill An alloy containing 5.04% Ni, 1.97% Al, 0.93% Co, 0.17% Mn, balance Cu was cast, processed and tested in an identical manner to the procedures of Example I. The weight loss results and corrosion rates plus localized corrosion observations are shown in Figures 1 and 2 and Table I, respectively.

It can be seen that the Alloy of Example Ill has a lower initial and steady state corrosion rate than Alloy 706 in potable water, together with freedom from localized corrosion. The 90-day corrosion rate of this alloy in brackish water is significantly less than the corrosion rate of Alloy 706.

Example IV An alloy containing 4.98% Ni, 1.98% Al, 1.00% Fe, 0.88% Co, 0.20% Mn, balance Cu was processed in a similar manner as shown in Example I with the exception that the alloy strip at 0.120" gauge was annealed at 8500C for 1 5 minutes and the alloy strip at 0.060" gauge was annealed at 850"C for 10 minutes. Alloy 706 was cast and processed in a similar manner with the exception that the strip was annealed at 8000C. These alloys were tested against each other and the weight loss was measured both in New Haven tap water and in synthetic brackish water. The results are shown in Figures 3 and 4.

It can be seen from Figure 3 that the performance of the alloy of the present invention is equivalent to the performance of Alloy 706. It can be seen from Figure 4 that the corrosion resistance of the alloy of the present invention in brackish water is significantly greater than the corrosion resistance of Alloy 706.

Table I Corrosion Rates and Localized Corrosion Observations 30-Day Corrosion 90-Day Corrosion Maximum Pit Depth Alloy Composition Rate (mpy) Rate (mpy) at 90 Days lmilsl A* B** A* B** A* B** Cm+5.21 Ni+2.05AI+ 1.08Fe+0.16Mn 0.05 0.46 0.02 0.33 0 3 Cu+4.88Ni+2.02AI+ 1.04Fe+.18Cr+.17Mn 0.04 0.44 0.02 0.29 0 3 Cu+5.O4Ni+ 1 .97Al+ 0.93Co+0.17Mn 0.09 0.20 0.04 0.09 0 2 Alloy 706 0.12 0.31 0.06 0.18 0 2 A* -New Haven Potable Water B**-Synthetic Brackish Water As can be seen from these examples, the alloy system of the present invention provides equivalent or greater corrosion resistance results than commercial Alloy 706 in both potable water and brackish water testing. This alloy system is intended as a lower cost replacement for Alloy 706 generally in fresh water applications. At present, Alloy 706 is not economically competitive with such materials as 304 stainless steels. Reduction of the nickel content and thus reduction of the cost brought about by the alloy system of the present invention without sacrificing corrosion resistance properties produces an alloy with much more favorable economics to those contemplating the use of copper alloys in tubing applications.The alloy of the present invention may also be utilized in various other applications, such as those applications which utilize the material for its strength properties or those which utilize the material for its pleasing appearance. For example, the alloy of the present invention may be useful as construction material and may also be useful in furniture or decorative applications. Various other uses of this alloy system will depend upon the particular property or properties desired by the fabricator in the final product.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. Thecpresent embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.

The alloy system as particularly disclosed herein provides increased resistance to corrosion in potable and brackish water applications compared to commercially available corrosion resistant alloys; exhibits a low initial corrosion rate to avoid soluble copper release to the environment on start up of tubing systems; and moreover retains single phase properties within the alloy structure after processing to increase corrosion resistance properties.

Claims (10)

Claims

1. A corrosion resistant single phase alloy which is particularly useful in tubing applications, characterized in that the elements present in said alloy consist essentially of 3.0 to 7.5% by weight nickel, 1.0 to 4.0% by weight aluminum, up to 3.0% by weight iron, 0.001 to 1.0% by weight manganese, balance copper.

2. An alloy according to claim 1 characterized in that up to 1.0% by weight chromium is also present in said alloy.

3. An alloy according to claim 1 characterized in that up to 3.0% by weight cobalt is also present in said alloy, provided that the maximum iron plus cobalt addition to said alloy is 4.0% by weight.

4. An alloy according to claim 1 characterized in that 0.01 to 2.0% by weight for each of the elements selected from the group consisting of arsenic, antimony and phosphorus, or combinations thereof, is also present in said alloy.

5. A corrosion resistant single phase alloy which is particularly useful in tubing applications, characterized by said alloy consisting essentially of 3.0 to 5.4% by weight nickel, 1.0 to 4.0% by weight aluminum, 1.0 to 2.0% by weight iron, 0.001 to 1% by weight manganese, balance copper.

6. A process of producing a wrought copper base alloy which is particularly useful in applications requiring corrosion resistance, said process characterized by the steps of: (a) providing a cast alloy consisting essentially of 3.0 to 7.5% by weight nickel, 1.0 to 4.0% by weight aluminum, up to 3.0% by weight iron, 0.001 to 1.0% by weight manganese, balance copper; (b) homogenizing said alloy at a minimum temperature of 5000C for at least 1 5 minutes; (c) hot working said alloy at a temperature of at least 4000 C; (d) rapidly cooling the hot worked alloy to insure a solid solution microstructure within the alloy; and (e) cold working said alloy at a temperature below 2000 C.

7. A process according to claim 6 characterized in that said cold working is performed in cycles with intermediate annealing from 525 to within 500C of the solidus temperature for the particular alloy from 10 seconds to 24 hours and said alloy is rapidly cooled after annealing to retain a single phase microstructure.

8. A process according to claim 6 characterized in that said rapid cooling is a water quench.

9. A process according to claim 6 characterized in that up to 1.0% by weight chromium is also present in said alloy.

10. A process according to claim 6 characterized in that up to 3.0% by weight cobalt is also present in said alloy, provided that the maximum iron plus cobalt addition to said alloy is 4.0% by