Effect of Composition on the Corrosion Behavior of Stainless Steels Brazed with Silver-Base Filler Metals An electrochemical galvanic action between coexisting phases is responsible for the corrosion of filler metal and interface BY T. TAKEMOTO AND I. OKAMOTO ABSTRACT. The effect of composition of been known to fail by interfacial corro­ improve the corrosion resistance of stain­ silver-base filler metals and austenitic sion. Sistare et al. tested the corrosion less steel brazed with silver-base filler stainless steels on the corrosion of stain­ resistance of Type 430 stainless steel metals. However, even the fundamental less steel joints brazed with silver-base joints (brazed with silver-base filler met­ aspects concerning the effect of compo­ filler metals was investigated in aqueous als) in running tap water and paid atten­ sition of Ag-Cu-Zn ternary filler metals sodium chloride solution. The depth of tion to the corrosion at the braze inter­ and stainless steel composition and also corrosion in the filler metal increased with face (Ref. 1). They considered the interfa­ the improvement mechanism by nickel the increase of zinc content in filler metal cial corrosion was similar to the crevice addition on the corrosion resistance of and (Ni + Cr) content of base metal. The corrosion which was accelerated by the stainless steels brazed with silver-base interfacial corrosion was promoted by formation of an oxygen concentration filler metals has not yet been clarified the increase of silver content in the filler cell. They used 40 kinds of silver-base sufficiently. metal. filler metals and recommended several The joint is usually made by the aid of The phenomena are explained by the relatively corrosion resistant filler metals flux to remove the passive oxide films galvanic cells such as the stainless steel containing nickel. and thereby permit intimate contact bulk surface/a-Cu phase in filler metals, Kawakatsu (Ref. 2) evaluated the cor­ between filler and base metal. Hence, the and the arAg phase/active stainless steel rosion resistance of Type 430 and 304 stainless steel is brazed to filler metal after at the braze interface. The brazing flux stainless steel joints brazed with silver- removal of the protective passive film leaves the active layer at the braze inter­ base filler metals. He did this by measur­ without repassivation; this leaves the thin face by the removal of protective passive ing the decrement of tensile strength active layer in stainless steel base metal at films; this is believed to be one of the after immersion in various acids and alka­ the interface. In addition to this, at the causes of interfacial corrosion. line aqueous solutions. This work pointed braze interface, various phases with dif­ A nickel-depleted ferrite phase is out that a nickel-rich layer was formed ferent electrochemical properties such as observed at the interface brazed with along the interface in nickel bearing BAg arAg, a-Cu, and austenite (passive sur­ nickel-free filler metals; however, the for­ filler metal joints; it stated that the layer face and active interface) coexist; this mation of a nickel-depleted zone is pre­ was responsible for the improvement of suggests galvanic corrosion cells may vented by the use of filler metals with 2% corrosion resistance. Other work also function among them. Ni. Addition of nickel to filler metals is referred to the advantageous effect of This paper describes the effect of Ag- found to prevent the formation of the nickel additions to filler metals on joint life Cu-Zn ternary silver filler metal composi­ less corrosion resistant nickel-depleted (Ref. 3) and dezincification in sea water tion and stainless steel composition on zone. (Ref. 4). The use of zinc-free, nickel- the corrosion behavior of brazed joints. It bearing filler metal is also found to be does so by considering the metallurgical beneficial (Refs. 5,6,7). introduction factors and electrochemical galvanic The selection of appropriate combina­ effects. The improved mechanism result­ Stainless steel joints brazed with silver- tions of silver-base filler metal and stain­ ing from nickel addition to silver filler base filler metals have been widely less steel appears to be important to metals was also investigated. employed for piping, industrial heat exchangers and other conventional joints. The joints are expected to be corrosion resistant in the atmosphere Table 1—Chemical Composition of Filler Metals, Wt-% containing moisture, aggressive gases and chloride ions. However, these joints have Brazing temperature Paper presented at the 15th International Filler metals Ag Cu Ni Zn °C CF) A WS-WRC Brazing and Soldering Conference held in Dallas, Texas, during April 10-12, BAg-6 50.50 34.90 Balance 800 (1472) 1984. BAg-5 45.11 30.53 Balance 800 (1472) CF-12<a» 40.47 30.38 Balance 800 (1472) T. TAKEMOTO is Research Instructor and I. BAg-4 39.89 30.86 2.05 Balance 830 (1526) OKAMOTO is Professor, Welding Research Institute, Osaka University, Osaka, Japan. (a) Laboratory number. 300-s | OCTOBER 1984 Table 2—Chemical Composition of Stainless Steel Base Metals, Wt-°o Element'3' Type stainless steel base metal Ni Cr Mn Mo Fe 304 8.71 18.04 0.06 0.61 1.04 0.029 Balance 304L 10.29 18.17 0.014 0.65 1.60 0.030 Balance 321 9.10 17.15 0.06 0.66 0.96 0.031 0.31 Balance 316 11.80 17.07 0.05 0.80 1.02 0.025 2.65 Balance 309 14.29 22.59 0.06 0.70 1.56 0.040 Balance 310S 19.80 24.64 0.06 0.73 1.68 0.024 Balance (a) S-0.001 to 0.009. Experimental Procedures 0.4 mol/liter NaCI + 0.005 mol/liter shown in Fig. 2. The BAg-6 consisted of CuCb • 2H20 aqueous solution; the tem­ a-Cu primary phase and eutectic struc­ The chemical compositions of filler perature was maintained at 25°C (77°F) ture (arAg + a-Cu). BAg-5 and BAg-4 metals and stainless steels are presented by a controlled water bath. After immer­ consisted mainly of a-Cu primary phase in Tables 1 and 2, respectively. The sion for certain days in test solution, and eutectic structure; however, in many machined dimensions (in mm) of the specimen cross sections were examined instances very small amounts of /? phase stainless steel base metals were: thick­ by an optical microscope and a profile is observed by x-ray diffraction analysis. ness—8, width—12, and length-60 projector with digital read-out facility. The microstructure of CF-12 is markedly (0.315 X 0.47 X 2.36 in.) with a center Metallurgical studies using an x-ray dif- different from others due to the exis­ groove of 2.5 mm (0.1 in.) radius. fractometer and scanning electron micro­ tence of (3 phase. Continuous crystalliza­ The filler metals and commercial braz­ scope with an energy dispersive x-ray tion of a-Cu primary phase at the braze ing flux were put in the groove. They analyzer, and electrochemical studies interface is dominant in BAg-4 brazed were heated to the brazing temperature were also carried out. specimens. in an air atmosphere furnace and held for The morphology of corrosion can be 1 min and then air cooled. Prior to the Experimental Results grouped in three types —Fig. 3. All speci­ brazing, the materials were rinsed in an mens underwent corrosion in the filler Microstructure and Corrosion Morphology acetone-ultrasonic bath. After brazing, metal and interface. The former was the the specimens were rinsed in warm The microstructure of filler metals is selective dissolution of a-Cu phase in filler water to remove the flux residues. The brazed specimens were then cut to 15 mm (0.59 in.) lengths and polished with no. 600 emery paper and subjected to immersion type corrosion test —Fig. 1. The specimens were designed to elimi­ nate voids at the brazed interfaces. Also, the area where filler metal thinly spread was eliminated by polishing. The speci­ x mens made it possible to measure the » S i.* ^p. ; Jfe;P A, 'mi.. interfacial corrosion length and corrosion &i* :*//i' fI Vl. ** ** depth of filler metal, respectively, and 304 304 were useful to discussion of the corrosion mechanism. y- a <% i BAe-4 'AA^ Corrosion tests were carried out using *5C *.) 12 j • ,T» 2.5R (01R) 30^ Fig. 2 — Microstructure of brazed specimens, Type 304 stainless steel base meta! X500 \ Cut after corrosion test ^" For corrosion test mm { in) ® ® © Fig. 1—Shape and size of stainless steel (A), Fig. 3-Schematic illustration of corrosion, interfacial corrosion at stainless steel side (A), both and corrosion test specimen (B) stainless steel and filler metal (B), and dominant corrosion of filler metal (C) WELDING RESEARCH SUPPLEMENT 1301-s 1700 3300 >4Q00 Base >4000 3 1000 g 6 b metal: SUS30A 6 CJ) c c o \n o L- k_ 500 o o o rd a*L—. Cb CF-12 C 10 20 30 Ti me , day Fig. 4 — Corrosion depth of filler metals brazed with Type 304 Fig 5 —Effect of filler metal on interfacial corrosion length stainless steel metals, similar to the dezincification of rich filler metal had superior corrosion on the corrosion depths of BAg-4 and 5 is a-brass (Refs. 8,9), and proceeded almost resistance compared to zinc-rich. Owing represented in Fig. 6. BAg-4 is more homogeneously from its surface. to the less corrosion resistant /3 phase corrosion resistant than BAg-5 on all base The corrosion path in the latter was (Refs. 8, 10, 11, 12), CF-12 showed deep metals.