Chromium Containing Amorphous Brazing Foils And Their Resistance to Automotive Exhaust Gas Condensate

Transcription

1 Chromium Containing Amorphous Brazing Foils And Their Resistance to Automotive Exhaust Gas Condensate Thomas Hartmann and Dieter Nuetzel Vacuumschmelze, Gruener Weg 37, Hanau, Germany Manuscript published in: Proceedings of the 5 th International Brazing and Soldering Conference April 22-25, 2012, Las Vegas, Nevada, USA ISBN-13:

2 IBSC 2012: Proceedings from the 5th International Brazing and Soldering Conference April 22 25, 2012, Las Vegas, Nevada, USA R. Gourley and C. Walker, editors Copyright 2012 ASM International All rights reserved Chromium Containing Amorphous Brazing Foils and Their Resistance to Automotive Exhaust Gas Condensate Thomas Hartmann and Dieter Nuetzel Vacuumschmelze, Gruener Weg 37, Hanau, Germany Abstract Brazed stainless steel components for exhaust gas applications are well established and extensively used in the EU, US and Japanese vehicle industry in order to reduce emission levels. The launch of the cooled exhaust gas recirculation (EGR) technology in countries with poor fuel quality such as parts of South America and Asia requires materials with an enhanced corrosion resistance. Especially the higher Sulfur content within these low quality fuels leads to an increased risk of failure due to corrosive damage if the base and brazing materials are not adapted. Quite recently, the German Association of the Automotive Industry (VDA) set a new standard and testing procedure for corrosion testing of metallic materials exposed to exhaust condensates. This paper analyzes the corrosion behavior of brazed samples joined with different Ni-Cr and Fe-Ni-Cr base amorphous brazing foils as well as the recently developed Ni-Cr-Si-P foils. A range of different test condensates, including the new VDA test standard, were used for evaluation of the corrosion resistance. Introduction At the end of the 19 th century the German engineer Rudolf Diesel developed the first compression-ignition engine, the diesel engine. Due to the high compression ratio diesel engines generally possess the highest thermal efficiency of any combustion engine. Even today modern diesel engines have a much better fuel efficiency and higher torque capacity than gasoline engines. In the last decades a lot of efforts were also made to reduce the emissions of these engines. Several components like Diesel Particulate Filters (DPF), Diesel Oxidation and Selective Catalytic Reduction (SCR) catalysts as well as Exhaust Gas Recirculation (EGR) systems were established to reduce emissions in order to meet the more and more stringent emission levels set by governments around the world. Due to the requirements 394 related to corrosion resistance as well as mechanical stability at high temperatures these components were typically made out of stainless steel and use high temperature brazing for joining. Figure 1 shows for example a fully brazed flat tube EGR cooler. ABF Figure 1: Fully brazed exhaust gas recirculation cooler using Amorphous Brazing Foils for joining For joining these exhaust gas components, especially EGR coolers, the use of Amorphous Brazing Foils (ABFs) is becoming more and more common. The increasing use of ABFs is based on distinct technological and commercial advantages of those foils in comparison to powder and paste brazing materials [1-3]. The foil form allows a high degree of process automation of such components and leads to beneficial joint properties. Metallurgical processes during joint formation are predictable and well understood [4-6]; furthermore the mechanical properties of these joints meet the requirements of various applications. Additionally, new foil compositions with properties especially adapted to the requirements of exhaust components were launched to the market [7,8,].

3 The chemical compositions of corrosion resistant ABF s are mainly Ni-Cr or Ni-Fe-Cr based. Additions of Silicon, Boron and Phosphorous lower the melting point and play an important role in formation of the amorphous structure during the production process using the melt spinning technology. Due to the high corrosive load of exhaust gas environments, the corrosion resistance of brazed stainless steel joints is of significant importance for their use in automotive exhaust gas lines. These systems are sophisticated constructions consisting of various sections whereby each area requires different material properties. Considering corrosion resistance, exhaust lines may be divided into three major parts [9]: The front part (manifold pipes, catalytic converter, EGR cooler), subjected mainly to high temperature oxidation, sometimes wet corrosion by fuel condensates (internal parts) The center section of the exhaust system (centre muffler, connecting pipes) subjected to high temperature oxidation (internal parts) and wet corrosion due to de-icing salt contaminations (external parts) The rear part (rear muffler), exposed to lower temperatures and subjected mainly to wet corrosion by fuel condensates (internal parts) and wet corrosion due to de-icing salt contaminations (external parts). Brazed components like the EGR cooler are typically located in the front part of the exhaust system where the most critical corrosion is mainly caused from the high temperature and the exhaust condensate attacking internal structures of the brazed components. Due to the very low chloride content of diesel fuels (single-digit ppm values) their concentration found in exhaust gas condensates are also rather low. Typical values are below 80 ppm; and mostly well below 50 ppm [13]. On the other hand the outer surfaces of the rear section of exhaust systems are exposed to a corrosive environment due to the use of salt (NaCl) for deicing. To simulate this corrosive load on external structures of exhaust parts some test condensates contain up to 1000 ppm chloride [9-11]. Brazed joints are mainly located in internal sections of the front part and the center area of the exhaust systems, e.g. EGR coolers or catalytic converters, where brazed joints are usually not exposed to high Chloride containing environments. The corrosiveness of the condensate is therefore mainly defined by the type and quality of the diesel fuel used 395 for the engine. The most common diesel fuel is a specific fractional distillate of petroleum fuel oil. Since 2003, almost all diesel fuel available in Europe, and since 2007 in the US, is the Ultra-low sulfur diesel (ULSD) which contains substantially low Sulfur contents. The limit for Sulfur was set to 10 ppm for EU, and 15 ppm for the US regulations. The chloride content of ULSD is typically below 10 ppm. During combustion of diesel fuel, corrosive gases containing Sulfur and Nitrogen are formed. Due to specific ambient conditions like temperature and humidity these corrosive gases react with moisture and form highly corrosive agents, especially sulfuric acid [12,13]. The sulfuric acid in diesel exhaust gas is primarily formed by a two step process describes as follows: Gaseous SO 2 reacts with oxygen: (1) 2SO 2 + O 2 2SO 3 SO 3 reacts with moisture or water vapor present in the exhaust stream: (2) 2SO 3 + H 2 O H 2 SO 4 Due to the low Sulfur content of ULSD fuel the exhaust gas and their extracted condensate are less aggressive related to corrosion compared to high Sulfur diesel fuels. Outside the US or the EU the Sulfur level in diesel fuel commonly far exceeds 50 ppm. For example, in rural areas of Argentina and Brazil diesel fuel contains between 500 and 2000 ppm Sulfur [17] whereas it is limited to 50 or 500 ppm in metropolitan areas. Since 2002 China has also limited the Sulfur content in diesel fuel to 2000 ppm with reduced limits of 500 ppm applied for certain cities. Exhaust gas condensates from these fuels are significantly more aggressive, especially due to their higher content of sulfuric acid. Due to the higher corrosiveness of exhaust products caused from high Sulfur containing low-quality fuels, it becomes necessary to reconsider the material selection in particular for EGR products, as soon as they become essential for diesel cars in these countries. The scope of this paper was to investigate the corrosion resistance of various ABF/stainless steel joints to aggressive high sulfur diesel exhaust gas condensates. Based on the results, a recommendation for ABFs able to withstand these harsh environments was given. Methods To evaluate the resistance of exhaust system components to condensate, the vehicle and component manufacturers most frequently conduct accelerated

4 corrosion tests between 200 and 1000 hours duration [9-11,13-15]. They are typically carried out as permanent or intermittent immersion tests in aqueous acid solutions. The composition of these agents contains mineral acids, like sulfuric and nitric acid, as well as organic acids, like formic or acetic acid. The ph value of these agents is between 1 and 4. For the simulation of practical conditions, intermittent immersion tests are typically combined with alternating warm ageing periods at defined elevated temperatures. Quite recently the German Association of the Automotive Industry (VDA) set a new standard and testing procedure for corrosion testing of metallic materials exposed to exhaust gas condensate [14]. This standard was primarily used as basis for the enclosed investigation. It consists of daily and weekly cyclical immersions followed by regular recurring phases. Details are given in Table 1 and Figure 2. Within this VDA standard a moderate, organic acid based (K2.1) and an aggressive, high Sulfur containing, mineral acid based (K1.1) condensate is given. To investigate the influence of de-icing salt on the corrosion performance of external parts of the exhaust line rear section a chloride content of 1000 ppm can be added to every type of condensate (VDA condensate K1.2 and 2.2). The condensates B1 and C1 match the condensates K1.1 and K1.2 of the VDA test. All condensates used within the investigation are listed in Table 2. For examination of the corrosion behavior of internal areas of the exhaust systems (for example EGR coolers), condensate types A and B with chloride concentrations ranging from 10 to 250 ppm and varying concentrations of sulfuric and nitric acids were used. (a) Table 2: Composition of exhaust gas condensates Test condensate ph H 2 SO 4 [ppm] (b) Figure 2: Brazed specimen placed in a test chamber with defined position to the condensate level, (a) semi immersed (b) in the vapor phase above condensate level HNO 3 [ppm] Cl - [ppm] A B B B B C Table 1: Sequence of the weekly test procedure of a condensate corrosion test according along VDA Total test duration: 6 weeks. Day Operation T [ C] Runtime [h per day] 1 (a) Sample evaluation - - (b) Heat aging (a) Semi immersion 50 6 in condensate (see Figure 2, a) (b) Drying 80 2 (c) Aging in condensate vapor (see Figure 2, b) Figure 3: Brazed specimen for VDA Test 396

5 Specimen For corrosion testing of brazing joints a sample geometry was chosen to assure an (i) adequate number of brazed joints, (ii) reliable brazing results and (iii) sufficient durability for sample handling and preparation. A sketch of the specimen is shown in Figure 3. The bore hole ( 6 mm) is used to place the sample in a defined, semi-immersed position in the test condensate (see Figure 2). With respect to the corrosiveness of the test condensates an AISI 316L stainless steel was selected as base material. A sheet of ABF (20 mm x 40 mm x 0,05 mm) was placed between the rods and the base plate prior to brazing. Brazing was carried out in a vacuum furnace with a vacuum level better than 1x10-3 mbar. Individual brazing parameters were set depending on foil composition (see Table 3). Metallographic analysis was done from the submerged part of the sample as well as from the part having exclusively contact to the vapor phase of the condensate. Furthermore the total mass loss of the samples was determined. Materials Chromium containing Nickel based brazing foils are widespread used to join components for automotive exhaust gas applications. These brazing alloys typically contain amounts of Silicon, Boron and Phosphorous in a total value of 7-11 wt. %. They are essential for lowering the melting range of the Ni-Cr matrix and for reducing the surface tension of the liquid brazing alloy in order to achieve good wetting and flowing behavior on the stainless steel substrate. On the other hand, the Silicon, Boron or Phosphorous containing phases in the brazing joint or especially the interfacial area of surrounding phases are frequently the origin of corrosive attacks. The reason for this can be an insufficient Chromium content of the whole specific phase or a Chromium depletion of the phase boundaries due to a substantial difference in chromium concentration of the involved phases. Depending on the type and amount of the melting point depressants and the presence of corrosion inhibiting elements like Chromium, Iron, Molybdenum or Copper, the investigated brazing foils may be more or less sensitive to the exhaust gas condensates. The selection of the tested ABFs includes compositions that are widely used for joining metallic exhaust parts. The chemical compositions and their brazing parameters are listed in Table 3. This selection contains ABFs with different Chromium contents of the traditionally Ni-Cr-Si-B alloys (VZ2120, VZ2111, VZ2150) as well as newly developed foils of the Ni-Cr- Si-P (VZ2170, VZ2177) and the Fe-Ni-Cr-Si-B (VZ2106, VZ2099) group. Vacuumschmelze GmbH & Co KG developed these compositions to optimize the technical and economical performance of brazed exhaust components [1,7,8]. Table 3: Chemical compositions of brazing foils and base material and selected brazing parameters Brazing alloy ABF designation ISO Chemical composition [wt.%] Ni Fe Cr Mo Cu Si P B Brazing conditions VZ2120 Ni 620 Bal VZ2111 Ni 610 Bal VZ2150 Ni 660 Bal VZ2170 Bal VZ2177 Bal 25 X X X VZ VZ T [ C] t [min.] Base Metal Chemical composition [wt.%] Fe Ni Cr Mo Mn Si C AISI 316L Bal Bal <2 < 1 <0,03 397

6 Results The corrosion behavior of the different brazing alloys in several condensates can be seen in Figure 4 and 5 where their mass loss and their visual appearance are shown. Based on the obtained results, the alloys can be classified into three groups: (1) ABFs containing > 2 wt% Boron (VZ2120 and VZ2111) The total mass loss (Figure 4) exhibits noticeable high values for the standard brazing alloys VZ2120 (Ni 620/BNi-2) and VZ2111 (Ni 610/BNi-1a) out of the Ni- Cr-Si-B group. This result was confirmed for all tested condensates. Cross-sections from the submerged parts of these samples show a critical uniform corrosive attack to the entire joint area (See arrows on figure 5a and 5b) for all type B condensates. VZ2120 joints in particular are very critical because the complete fillet area disappeared. The type A condensate with less concentration of sulfuric- and nitric acid leads to a slightly better result. Metallographic examination of the specimen exposed to the vapor phase of type B condensates discloses also significant attack of the central joint area and the interfacial zone to the base metal. Slightly better, but still very critical results are obtained for the VZ2111 joints. The insufficient corrosion resistance of these standard brazing materials is mainly caused from the relatively VZ2120 VZ2111 VZ2150 VZ2170 VZ2177 VZ2106 VZ ,2 0,4 0,6 0,8 Mass loss [%] Condensate B1 Condensate C1 Figure 4: Total mass loss of the brazed samples for condensates B1 and C1 low Chromium content (VZ2120: 7 wt. %, VZ2111: 13 wt. %) combined with a quite high Boron content of around 3 wt.%. This leads to an excessive formation of Cr x B y compounds in the brazing joint and the adjoining base metal area. These Cr x B y compounds contain high concentrations of Chromium which is no longer available for improving the corrosion resistance of the surrounding phases. It can be seen that the corrosive attack starts primarily at the interface of these phases as well as at the interface and adjoining areas of the stainless steel. In these areas typical intercrystalline Cr x B y precipitations at grain boundaries in the base material are formed [2,4,6,7]. (a) VZ2120 (b) VZ2111 (c) VZ2150 (d) VZ2170 (e) VZ2099 (f) VZ2106 Figure 5: Metallographic examination of brazed joints after completion corrosion test in condensate type B1. Cross sections were taken from the submerged area of the samples. 398

7 Table 4: Visual appearance of brazed joints microstructure (according to Figure 5) after the corrosion test depending on brazing foils, types of condensate and for liquid and vapor phase of the test condensates. Alloy Media A1 B1 B2 B3 B4 C1 VZ2120 VZ2111 VZ2150 VZ2170 VZ2177 VZ2106 VZ2099 vapor fluid vapor O fluid fluid O O O O O O fluid O O fluid O O O O O fluid O O O O O O fluid O O O O O = No or marginal corrosive attack = Initiating or selective corrosion is visible = Critical corrosive attack of wide areas (2) (2) ABFs containing > 1.5 wt.% Phosphorous (VZ2099 and VZ2170, VZ2177) The joints made with ABF foils out of the (Ni,Fe)-Cr- Si-B-P system shows a very good resistance to aggressive condensates containing high amounts of sulfuric and nitric acid. It was found that corrosive attacks occur only when the amount of chloride exceeds certain limits. These limits are found to be different for each alloy but they depend mainly on the Chromium content of the brazing material and on the brazing parameters [1,16]. The morphological structure of these joints consists of several different phases. Beside the (Ni,Fe)-Cr matrix phase the major volume fraction consists of several dendritic (Ni,Fe) x -Cr y -P z phases. It can be seen that the Chloride-induced selective corrosion starts primarily at crystal boundaries of these intermetallic (Ni,Fe) x -Cr y -P z phases. During the formation of these compounds the 399 Chromium content of the interface area will be reduced. This promotes local attacks by corrosion when the Chloride content exceeds a critical level. It seems that different to the intermetallic Cr x B y phases in Ni-Cr-Si-B alloys the interface has still a notable content of Chromium because in reducing acidic medias the joints show an excellent corrosion resistance. The higher resistance of the interface areas may be caused from the generally higher Chromiumcontent of the Ni-Cr-Si-P alloys VZ2170 and VZ2177 ( 21 wt. %) compared to the Ni-Cr-Si-B alloys VZ2120 and VZ2111 ( 13 wt. %). Different to the formation of intermetallic Cr x B y phases in Ni-Cr-B-Si alloys which are formed in the joint as well at the grain boundaries of the adjoining base material, the formation of the (Ni,Fe) x -Cr y -P z phases in Ni-Cr-Si-P alloys is limited inside the brazing joint. Metallographic analysis from the vapor phase area did not show any corrosive attack for all tested condensates, even if the chloride content was 1000 ppm. The joints of the (Ni,Fe)-Cr-Si-B-P brazing foils exhibit significantly better resistance to vapor of exhaust condensate than it can be found for VZ2120 or VZ2111 joints. Table 4 summarizes the results for the different brazing foils and test condensates. (3) ABFs containing < 2 wt% Boron (VZ2150 and VZ2106) As expected VZ2150 (Ni 660/BNi-5a) with a relatively high Chromium content of 18 wt.% combined with a very low Boron content of 1.15 wt.% exhibits an excellent corrosion performance regardless which condensate was used. Although the Chromium content of VZ2106 is well below the value of VZ2150 (18 wt. % vs wt. %) comparable values of mass loss was found. This result will be confirmed by the metallographic analysis which shows no corrosive attack at VZ2106 joints (see Figure 5f) even if the chloride content is 1000 ppm. The reason is the adopted chemistry of this ABF which was optimized for automotive exhaust systems. The presence of Iron in a Ni-Cr matrix improves the resistance to Sulphur and Chloride containing medias. Additions of Molybdenum enhance the resistance towards non-oxide type acids like sulphuric and hydrochloride acids. Molybdenum, even in moderate amounts, assists the formation of a stable oxide layer and therefore improves the resistance against Chloride ions, Sulphur containing medias, crevice and pitting corrosion. Copper, even in small amounts, improves the resistance in hydrochloric acids. Small additions of Copper lower furthermore the active and passive current density and so on supports the effect of

8 Chromium in the described alloy. By lowering the active current density an improvement in the corrosion resistance can be expected in oxidising and reducing agents [7,15,19]. Conclusion The specific corrosion performance of the investigated ABFs against high Sulfur containing exhaust gas condensate can be summarized as follows: The corrosion resistance of the traditional amorphous brazing foils of the Ni-Cr-Si-B system VZ2120 and VZ2111 appear insufficient to the liquid and the vapor phase of all tested high Sulfur containing exhaust gas condensates of type B and C. General corrosive attack was observed on the whole joint area. The ABFs of the (Ni,Fe)-Cr-Si-B-P system VZ2170, VZ2177 and VZ2099 exhibit very good resistance to high sulfur containing exhaust gas condensates and their vapor phase. The corrosion resistance of these alloys is superior to that of the traditional VZ2120 or VZ2111 alloys. The ABFs of the (Ni,Fe)-Cr-Si-B-P system are sensitive to higher chloride concentrations in high Sulfur containing acid condensates. Depending on the Chromium content of the brazing alloy and on the chloride concentration of the liquid media, a selective corrosive attack may occur on the boundaries of some Phosphorous containing phases. All ABFs of the (Ni,Fe)-Cr-Si-B-P system VZ2170, VZ2177 and VZ2099 exhibit very good resistance in contact with the steam phase of all tested condensates, even at a chloride content of 1000 ppm and a Sulfur content of 3000 ppm. The ABFs of the (Ni,Fe)-(Cr,Mo,Cu)-Si-B system VZ2150 and VZ2106 exhibit good corrosion resistance against high Sulfur containing condensates, even if chloride concentration reach excessive values of 1000 ppm. The ABF VZ2106 is a good example that additions of Iron, Molybdenum and Copper in an Ni-Cr matrix will improve the corrosion resistance significantly even if the Chromium content is on a relatively low level (11,5 % of VZ2106 vs. 18% of VZ2150). 400 These results confirm the fact that well designed modern ABFs are beneficial over traditional alloys. They are able to withstand the high corrosive load of a harsh automotive exhaust gas environment even if fuel quality is very low. In combination with the beneficial processing and the advantageous brazing characteristic of amorphous brazing foils their soaring use for modern exhaust gas application may become evident. Concluding remarks It is important to not that the enclosed results cannot substitute the examination of a component behavior under real application conditions. Accelerated corrosion tests often use higher concentrations and temperatures of the corrosive media, well above real operating conditions, to decrease the time of testing [15,18]. Because the effect of these tightened conditions on corrosion rates may be great, there is a danger that economic, yet suitable materials may be eliminated. Also if the test-condition/corrosion-rate relation is nonlinear an extrapolation of the results could lead to incorrect conclusions. Acknowledgments We would like to thank the following colleagues of VAC for their valuable assistance: Bernd Hain, Dr. Stephan Lassmann, Marina Lemcke, Gabriele Rodner and Harald Staubach. References [1] T. Hartmann, D. Nützel (2010) Nickel-Chromium Based Amorphous Brazing Foils for Continuous Furnace Brazing of Stainless Steel; Proceedings of the 9 th international Conference on Brazing, High Temperature Brazing and Diffusion Bonding, LÖT 2010, Aachen; DVS Berichte 263, p [2] A. Rabinkin (2003) Overview: brazing with (NiCoCr)-B-Si amorphous brazing filler metals; Science and Technology of Welding and Joining, Volume 9, No. 3, June 2004, S [3] M. Naka, I. Okamoto (1985) Amorphous Alloy and its Application to Joining, Trans. JWRI, Vol 14 (No. 2), Dec 1985, p [4] American Welding Society (1991) The Brazing Handbook; IBSN: [5] K. D. Partz (1981) Einfluss von Lötparametern auf die Festigkeit stumpfgelöteter Hochtemperaturlötverbindungen Parametercharakteristik der Lötsysteme B-Ni2, B-Ni5, B-Ni7 / , ; Technisch- wissenschaftliche Berichte der RWTH Aachen, Nr [6] E.A. Leone; A. Rabinkin; B. Sarna (2006) Microstructure of Thin- Gauge Austenitic and Ferritic Stainless Steel Joints Brazed Using Metglas Amorphous Foil; Welding in the World, Vol. 50 No. 1/2, 2006

9 [7] T. Hartmann, D. Nützel (2007) Iron containing brazing foils for joining of stainless steels; Proceedings of the 8 th international Conference on Brazing, High Temperature Brazing and Diffusion Bonding, LÖT 2007, Aachen; DVS Berichte 243, p [8] T. Hartmann, D. Nützel (2009) New amorphous brazing foils for exhaust gas applications; Proceedings of the 4 th International Brazing and Soldering Conference, IBSC 2009, Orlando, USA, p [9] C. Hoffmann, P. Guempel (2009) Pitting corrosion in the wet section of the automotive exhaust systems, Journal of Achievements in Materials and Manufacturing Engineering, Vol. 34, Issue 2, June 2009, pp [10] P. Guempel et.al. (2004) Simulation des Korrosionsverhaltens von nichtrostenden Staehlen in Pkw-Abgasanlagen, Automobiltechnische Zeitschrift, Vol. 106, Issue No. 4, p [11] Y. Inoue, M. Kikuchi (2003) Present and future Trends of Stainless Steel for Automotive Exhaust System, Nippon Technical Report No. 88, UDC :629:11 [12] M.D. Kass et.al. (2005) Assessment of Corrosivity Associated with Exhaust Gas Recirculation in a Heavy Duty Diesel Engine, SEA Technical Paper, DOI: / [13] R.L. Chance, R.G. Ceselli (1983) Corrosiveness of Exhaust Gas Condensates, SAE Technical Paper, DOI: / [14] VDA Specification (2010) Resistance of metallic materials to condensate corrosion in exhaust gas carrying components, Dokumentation Kraftfahrtwesen e.v. [15] B.D. Craig; D.S. Anderson (1995) Handbook of Corrosion Data 2 nd Edition; ASM International [16] T. Hartmann, unpublished results [17] Wikipedia [ [18] S.D. Cramer, B.S. Covino (2003) ASM Handbook Volume 13A Corrosion: Fundamentals, Testing and Protection; ISBN , S. 194, p. 420ff [19] U. Brill; (1990) Korrosion und Korrosionsschutz bei Nickel, Cobalt und Nickel- und Cobalt-Basislegierungen; Thyssen Krupp VDM Informationsschrift, S