Corrosion Testing of Silver-Brazed Joints in Tap Water

Chloride in tap water influenced the corrosion rates of two filler but had little effect on another

BY H. -D. STEFFENS, B. WIELAGE AND W. BRANDL

ABSTRACT. Three different silver-based over the next several years, at least in plumbing applications. This sum­ brazes (B-Ag 56 Cu-ln-Ni, B-Ag 60 Cu-Sn central Europe. The chemical aggressive­ marizes the corrosion testing of silver and B-Ag 72 Cu) were investigated with ness of these media will, of course, vary brazed stainless steel joints in tap water. respect to their corrosion behavior in tap widely, with pollution being heavy in water. As a result, it was found that the densely populated industrial centers and Experimental Materials chloride possesses a signif­ less pronounced in predominantly agri­ icant influence on the resistance of the cultural and more thinly populated areas. The material selected for the base joints produced with the filler metals B-Ag From this follows the necessity to adapt was Cr-Ni-Mo-Ti 18-10 stainless 56 In-Ni and B-Ag 60 Cu-Sn. While the materials more than is done today to steel. The brazing alloys used were B-Ag former can be used in water up to 50 mg these expected differences in corrosive 56 Cu-ln-Ni (620-730), B-Ag 60 Cu-Sn Cl~/L without showing knife-line attack, attack and to consider the use, in certain (600-720) and B-Ag 72 Cu (780)-Ta­ the latter is feasible for higher chloride areas, of materials with higher resistance ble 1. concentrations. For the braze B-Ag 72 to corrosion than the conventionally used Cu, it was determined that an increasing plumbing materials, such as and Production of Brazed Joints CT content does not associate with a galvanized steel. Particularly suitable are higher corrosion rate. Cr-Ni-Mo steels which can be used in For the production of brazed joints, water with a chloride- content of up seamless cold-drawn pipe with an inter­ to 1000 mg/L. nal diameter of 15 mm (0.6 in.) and a wall Introduction thickness of 1.5 mm (0.06 in.) was used, Basically, three methods are suitable brazing being done with braze fittings. The corrosion behavior of stainless for joining Cr-Ni-Mo-steel pipe: welding, The braze filler metal was available in steels and their brazed joints have been, brazing and the use of press fittings with form (thickness 1.2 mm/0.05 in.). and continue to be, the subject of nonmetallic seals. Rings were made from the wire for numerous investigations. Joining of thin-walled stainless steel pipe brazing. Brazing was done with a fluxing Due to the heavy increase in water by welding is a highly specialized joining agent in air for the first two brazes consumption and the accompanying technique which cannot be safely per­ mentioned and in a vacuum furnace for increasing pollution of effluents, the cor­ formed by a plumber at a construction site. the third. The joint clearance for the rosive aggressiveness of the waters car­ Press fittings of passive stainless steel with furnace braze was set at 0.05 mm, and ried in sanitary installations will increase nonmetallic seals are generally unsuitable for brazing in air, a joint clearance of 0.1 due to crevice corrosion at contact areas mm was used. between seal and steel pipe. The structure of the brazed joint pro­ An alternative to press fittings is silver- KEY WORDS duced with Braze B-Ag 56 Cu-ln-Ni 620- brazing of stainless steel pipes. This is also 730 is shown in Fig. 1. Corrosion Testing suitable for joining nonferrous materials, Silver-Brazed joints such as copper, and can be done on-site. It is composed of three phases. The Tap Water Corrosion However, the often unfavorable corro­ copper-rich a- in the brazed joint is Cr-Ni-Mo Pipe sion behavior of brazed joints has so far visible in the form of large-area, dark Pipe Corrosion argued against its practical use in house- regions and occurs at the base-to-braze Tap Water Piping interface as a coherently precipitated B-Ag 56 Cu-ln-Ni 620-730 phase. The matrix consists of silver-rich «i B-Ag 60 Cu-Sn 600-720 1—Chemical Composition of Brazing phase (large, light-colored regions in Fig. B-Ag 72 Cu 780 Alloys 1). The finely precipitated phase in the Vacuum Furnace Braze matrix is the Ag-ln /3i phase. Chemical Composition Figure 2 shows the structure of Braze Brazing % B-Ag 60 Cu-Sn 600-720 after brazing. It Alloy Ag Cu Sn Ni In also consists of three phases, the silver- H. -D. STEFFENS, B. WIELAGE and W. BRANDL rich matrix (light colored «i phase), a 0 are with the University of Dortmund, Dort­ B-Ag 56 Cu-ln-Ni 56 26 — 4 I4 phase (containing Ag and Sn) finely pre­ mund, West Germany. (620-730) cipitated in the matrix, and the copper- B-Ag 60 Cu-Sn 60 30 10 - — rich a phase. In contrast to Braze B-Ag 56 Paper presented at the 17th International A WS (600-720) Cu-ln-Ni, however, the a-phase has not Brazing Conference, held April 15-17, 1986, in B-Ag 72 Cu (780) 72 28 Atlanta, Ga. - — — been precipitated as a coherent seam at

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the braze-to-base interface. existing in pipelines in regard to variation of the chloride content was The eutectic structure of the silver- content and flow patterns, an experimen­ achieved with the aid of a micropump. braze B-Ag 72 Cu 780 is shown in tal device was used in which fresh water Prior to the test, the specimens were Fig. 3. flowed over the specimen at a constant ground and polished with paste rate —Fig. 4. to a 1 fim finish. Methods of Investigation Corrosion Testing Device To investigate the corrosion behavior Test Results of the brazed joint in Dortmund drinking The work cell of the corrosion testing Experimental Results in Tap Water water (Table 2), electrochemical tests device is made of polyvinylchloride. A were made. The influence of the chloride contact pin is provided whereby the The current versus potential content of the water on the polarization specimen is connected to the work elec­ curves of the Cr-Ni-Mo-Ti behavior of the brazed joints was investi­ trode via the anode rod. The auxiliary 18-10 stainless steel brazed with the gated. Chloride contents were varied electrode is arranged in a circle around three different silver brazes in tap water from 25 to 500 mg/L. the work electrode. Potential measure­ with a chloride content of 25 mg/L are In order to reproduce the conditions ment was made via the salt bridge probe shown in Figs. 5-7. For the purposes of with the aid of the reference electrode. comparison, the current density versus For all measurements, a saturated calomel potential curve of the base material is also electrode was used as the reference shown in these figures. The steady-state Table 2—Analysis of Dortmund Tap Water electrode. Observation of the specimen potentials of the brazed joints are, with­ during the test was made possible out exception, more positive than those Total CH 8.70 °dH through the Plexiglas pane by means of a of the base material. Appearance of the Carbonate hardness KH 5.90 °dH stereomicroscope. The electrolyte braze joint in its initial state at potential P-i pH value 7.76 flowed through the work cell from end alters during the polarization test at dif­ Electrical conductivity 381.00 MS/cm to end and the probe was positioned in ferent potentials referred to as P2, P3, P4 Cl~ content 38.80 mg/L the direction of flow. The rate of flow and P5. CC>2 content 2.00 mg/L was variable. Figure 8 shows the change in the O2 content 6.40 mg/L structure of the braze joint during the Temperature condition 15.00 °C The continuous, metered addition of chloride solution required for polarization test carried out on the Cr-

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z Fig. 8-Appearance of B-Ag 56 Cu-ln-Ni brazed joint during electro­ ui chemical polarization. A—Initial structure at —400 mV; B —corrosive S a. attack begins, +190 mV; C—brownish layer forms, +260 mV; D — O corrosion grows into a dense, firmly adhering layer, +400 mV

-tOOmV a P2 > 2SmV ->» P3« 65 mV x P. • 210 mV o as 5. 400 mV < -(.00 -300 -200 -100 0 100 200 300 00 Ul Potential ImVJ tn ui Fig. 7 - Polarization curve of Cr-Ni-Mo-Ti 18-10 stainless steel/B-Ag 72 as Cu joint in Dortmund tap water

WELDING RESEARCH SUPPLEMENT 1153-s 400 mV

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Fig. 9 —Appearance of B-Ag 60 Cu-Sn brazed joint during polarization. Fig. 10 — Appearance of B-Ag 60 Cu-Sn brazed joint during polarization. A — Original structure; B — initial corrosive attack; C — corrosion intensi­ A — Formation of second layer begins; B—growths continue; C —strong fies adhesion of layer, even after one minute of

Ni-Mo-Ti 18-10 stainless steel/B-Ag 56 The original structure of the braze (Fig. shown in Fig. 11 A. Cu-ln-Ni brazed joint. 9A, Point P-i) changes from about 50 mV In the eutectic structure, the copper- The initial structure of the brazed joint onward (Point P2 in Fig. 6). The a-Cu rich solution is preferentially (Fig. 8A) at -400 mV (Point PT on the phase sustains the initial corrosive attacked. In the further stages of the current density versus potential curve — attack —Fig. 9B. With increasing polariza­ current density versus potential curve, Fig. 5) begins to change from about +190 tion, up to about 225 mV (Point P3), there the current density drops. This drop is mV (Point P2), onward. There are indica­ is a massive rise in current density. The accompanied by the formation of a layer. tions of corrosive attack on the a-Cu incipient corrosion of the copper-rich The renewed steady rise in current densi­ phase in the joint and at the braze- phase intensifies and a layer begins to ty, which points to increased dissolution to-base interface - Fig. 8B. This in­ form from the a-Cu phase —Fig. 9C. At of the silver-rich phase, subsequently lev­ cipient corrosion grows as the anodic the highest current density of the polar­ els off slightly at about 210 mV. It is here polarization increases, which is associated ization curve (at about 290 mV, P4 in Fig. that a further layer begins to form starting with a steep rise in current density — 6) the formation of a second layer at the edge of the brazed joint (Fig. 11B) Fig. 5. begins —Fig. 10A. This layer covered the completely covering the brazed joint at With the fall of the current density at entire brazed joint at increasing polariza­ 400 mV. Extended polishing proved in­ P3 (Fig. 5) a brownish layer begins to form tion and exhibited no tendency to be capable of removing this layer, indicating (Fig. 8C), with the formation completed flushed off-Fig. 10B. Figure 10C illus­ how firmly adhered it was. at about +260 mV; the formation of this trates the strong adhesion of this layer In summarizing the test results of the layer starts from the a-Cu phase. The after a minute's polishing; a light-colored Dortmund tap water, one can state that renewed rise in current density from 260 covering layer (top layer) was formed, under these conditions of water compo­ mV onward (Fig. 5) is associated with an which had grown beyond the edge of sition and temperature, i.e., room tem­ attack on the silver matrix (a-pAg-rich the brazed joint. In contrast to Braze B-Ag perature (Table 2), all three brazes exhib­ phase). As the «rAg phase goes into 56 Cu-ln-Ni, only minor corrosive attack ited good stability, with Braze B-Ag 72 Cu solution, however, a dense, firmly adher­ was detected on the remaining struc­ performing best. With this particular ing layer grows in these areas of the ture. braze, the depth of attack was the low­ brazed joint —Fig. 8D. In polarizing the Cr-Ni-Mo-Ti 18-10 est, while the top layer, which forms in The appearance of the Cr-Ni-Mo-Ti stainless steel/B-Ag 72 Cu-brazed joint, the process, adheres firmly. No corrosive 18-10 stainless steel/B-Ag 60 Cu-Sn some initial corrosive attack was noted at attack was noted on the base metal. brazed joint during the polarization test is +25 mV (Point P2 in Fig. 7). The appear­ While Braze B-Ag 56 Cu-ln-Ni with the shown in Figs. 9 and 10. ance of the brazed joint at +25 mV is copper-rich a-phase does sustain some

154-s I APRIL 1989 z 5ui a. O > ui a x o DS < Ul tn uois 25 mV, P; 300 urr ui 5 a. O > uai o as < ui tn

z uSi a. B o _J 300 pm 210 mV, >ui Fig. 11 — Appearance of B-Ag 72 Cu brazed joint during polarization. fig. 12 — Appearance of B-Ag 56 Cu-ln-Ni brazed joint after polarization A —Initial corrosive attack; B — second layer forms at edges of joint in tap water X o as < Fig. 13 — Polarization Ul corrosive attack, no knife-line attack was tn curves of ui observed at the chloride ion concentra­ Cr-Ni-Mo-Ti 18-10 OS tions used in this work. stainless steel/B-Ag 56 Cu-ln-Ni brazed Ul Experimental Results in Tap Water with joints Different Chloride Contents a. With the stepwise increase of the chlo­ o _l ride content, Braze B-Ag 56 Cu-ln-Ni ui showed increased corrosive attack in the > Ul brazed joint (Fig. 12) and knife-line attack ua for chloride ion concentrations from as about 50 mg/L or higher. < Ul Figure 13 shows the current density tn versus potential curve for this brazed uais joint at chloride concentrations in Dort­ --^ mund tap water of 25, 100, 200 and 500 Zt- mg/L. -10-4 ui 2 The curves hardly differ from one a another up to chloride contents of 200 O mg/L. However, the appearance of cor­ rosion at the brazed joint is very diverse —Figs. 8D and 14. At 25 mg/L chloride, no knife-line attack could be detected at the braze- o to-base interface (Fig. 8D), and at 100 a: 25mg/l CI" as b:100mg/l CI" < mg/L chloride, the interface has been Ul c:200mg/l CI" tn partly dissolved, while at 200 mg/L chlo­ d: 5O0mg/l CI" ui ride, it is completely dissolved —Fig. 14. as This phenomenon is even more pro­ -400 -300 -200 -W0 0 100 200 300 LOO nounced at the maximum chloride con- Potential [ mV ]

WELDING RESEARCH SUPPLEMENT 1155-s Fig. 14 - B-Ag 56 Cu-ln-Ni after polarization in tap water Fig. 15-Surface layers formed on B-Ag 72 Cu joints in tap water tent of 500 mg/L-Fig. 14. For these to the corrosive attack on the silver-rich they vary in appearance, suggesting dif­ chloride concentrations, the current den­ phase and the layer formation proceed­ ferences in composition —Fig. 15. sities are, by comparison, considerably ing from it. Nevertheless, no preferential At intermediate chloride contents (100 higher-Fig. 13. attack could be detected at the braze- and 200 mg/L), only incomplete cover­ The corrosive attack of Cr-Ni-Mo-Ti to-base interface. age of the brazed joint occurs (Figs. 16B 18-10 stainless steel/B-Ag 60 Cu-Sn- With the joint made with brazing alloy and C), with the structure having been brazed joints increased with increasing B-Ag 72, a phenomenon was found, as attacked only slightly. chloride content, too. The attack on the yet unexplained. At low (25 mg/L) and Clarification of these phenomena, pos­ a phase, as well as the associated layer high (500 mg/L) chloride contents, firmly sibly attributable to the influence of chlo­ formation, undergo a shift towards more adhering layers form, which completely ride content on carbonate equilibrium, is negative potentials. The same also applies cover brazed joint. Yet, after polishing, the subject of further work.

Fig. 16 —Appearance of B-Ag 72 Cu brazed joints after polarization in tap water with different chloride contents. A — Corrosion completely covers the joint; B —incomplete coverage; C —coverage grows; D — layer firmly adheres across the joint Fig. 17 —B-Ag 56 Cu-ln-Ni brazed joint

156-s | APRIL 1989 Conclusion ure of the joint. of the covering layer that were found While the joint made with the Braze here, are to be clarified in further The Braze B-Ag 56 Cu-ln-Ni can be B-Ag 60 Cu-Sn is subject to stronger research work by correlating electro­ used in cold drinking water up to approx­ attack at rising chloride contents, no pre­ chemical tests and the results of plant imately 50 mg of chloride per litre. At ferred attack at the base-to-braze phase corrosion tests with surface-analytical higher chloride contents, increasing sus­ boundary takes place. Hence this braze, measuring techniques, especially Auger ceptibility of knife-line attack must be on the basis of the electrochemical tests, Electron Spectroscopy. Since this method expected. According to present findings, can also be used in drinking water with also allows the composition of the layer this susceptibility is not due to the con­ higher chloride contents. with depth to be determined (by incre­ stant precipitation of the less a The test results obtained on the joints mental removal of material by argon phase at the brazed joint, but rather to a made with the Braze B-Ag 72 Cu indicate sputtering), inferences as to the forma­ very thinly formed intermediate layer, that an increasing chloride content of the tion mechanisms of the covering layers which still requires exact identification — electrolyte, e.g., tap water, is not neces­ with regard to the polarization potential Fig. 17. The preferred attack of this inter­ sarily associated with a higher rate of and water composition may be mediate layer probably to the fail­ corrosion. The variations in the formation deduced.

WRC Bulletin 329 December 1987 Accuracy of Stress Intensification Factors for Branch Connections By E. C. Rodabaugh

This report presents a detailed examination of the stress intensification factor (SIF) formulations for perpendicular branch connections that are specified in American standard codes for use in the design of industrial and nuclear Class 2 and 3 piping systems. Publication of this report was sponsored by the Subcommittee on Piping, Pumps and Valves of the Pressure Vessel Research Committee of the Welding Research Council. The of WRC Bulletin 329 is $20.00 per copy, plus $5.00 for postage and handling. Orders should be sent with payment to the Welding Research Council, Suite 1301, 345 E. 47th St., New York, NY 10017.

WRC Bulletin 330 January 1988 This Bulletin contains two reports covering the properties of several constructional-steel weldments prepared with different welding procedures.

The Behavior of A588 Grade A and A572 Grade 50 Weldments By C. V. Robino, R. Varughese, A. W. Pense and R. C. Dias

An experimental study was conducted on ASTM A588 Grade A and ASTM A572 Grade 50 microalloyed steels submerged arc welded with Linde 40B weld metal to determine the fracture properties of base plates, weld metal and heat-affected zones. The effects of plate orientation, heat treatment, heat input, and postweld heat treatments on heat-affected zone toughness were included in the investigation. Effects of Long-Time Postweld Heat Treatment on the Properties of Constructional-Steel Weldments By P. J. Konkol

To aid steel users in the selection of steel grades and fabrication procedures for structures subject to PWHT, seven representative and high-strength low-alloy plate steels were welded by shielded metal arc welding and by submerged arc welding. The weldments were PWHT for various times up to 100 h at 1100°F (593°C) and 1200°F (649°C). The mechanical properties of the weldments were determined by means of base-metal tension tests, transverse-weld tension tests, HAZ hardness tests, and Charpy V-notch (CVN) impact tests of the base metal, HAZ and weld metal.

Publication of these reports was sponsored by the Subcommittee on Thermal and Mechanical Effects on Materials of the Welding Research Council. The price of WRC Bulletin 330 is $20.00 per copy, plus $5.00 for postage and handling. Orders should be sent with payment to the Welding Research Council, 345 E. 47th St., Suite 1301, New York, NY 10017.

WELDING RESEARCH SUPPLEMENT 1157-s