CORROSION RESISTANCE OF WROUGHT ALUMINUM ALLOYS UTILIZED IN ENVIRONMENTS CONTAINING AUTOMOTIVE EXHAUST GASES. Zita DROZDOVÁ

Transcription

1 CORROSION RESISTANCE OF WROUGHT ALUMINUM ALLOYS UTILIZED IN ENVIRONMENTS CONTAINING AUTOMOTIVE EXHAUST GASES Zita DROZDOVÁ Halla-Visteon Climate Control Corporation, Novy Jicin, Czech Republic, EU, Abstract Automotive heat exchange applications utilizing wrought aluminum alloys which must resist exposure to corrosive exhaust gasses are becoming more prevalent in reduced hydrocarbon emission systems. This work attempts to present a test method for and demonstrate the corrosive effects of a simulated acidic exhaust gas condensate on various brazed composite wrought aluminum automotive heat exchanger test samples. Immersion testing was performed on several long life aluminum alloys as well as several multiclad structures. The long life alloys suffered from inter-granular corrosive attack; however, the attack was effectively controlled by the presence of the protective brown band layer formed during the brazing of these samples. The multiclad structures were found to perform quite well by experiencing only moderate surface attack. The addition of oxygenation to improve the accuracy of this simulated environmental exposure testing is also discussed. Keywords: corrosion resistance; wrought aluminum; exhaust gas emissions; acid condensate; immersion test 1. INTRODUCTION Automotive manufacturers face growing challenges in meeting restrictions placed on the levels of substances contained within the exhaust gas emissions of their vehicles. In an attempt to meet the latest stringent European requirements for exhaust gas emissions, it is necessary to use highly sophisticated emission control systems that have demanding performance and durability requirements on their subcomponents. The development of the low-pressure exhaust gas recirculation (EGR) system was initiated in response to European Union aimed at reducing vehicle emissions. The current emission standard is known as Euro 5 (in place since 2008) establishes limiting values on exhaust gases containing: carbon monoxide (CO); hydrocarbons (HC); nitrogen oxides (NO x ); and particulate emissions (PM) [1]. In September of 2014, the stricter Norm Euro 6 limits will be imposed. Specific emission limits are provided in Table 1. Table 1. Limits for exhaust gas emissions according to Euro 5 and Euro 6 in brackets (red), PI=Positive Ignition, CI=Compression Ignition. Positive Ignition particulate mass standards apply only to vehicles with direct injection engines [1]. Category Class PI CI PI CI PI CI PI CI PI CI PI CI M - All (80) (170) 5 5 N1 I RM (80) (170) 5 5 II Reference mass Mass of carbon (RM) (kg) monoxide (CO) 1305<RM 1760 Mass of total hydrocarbons (THC) Mass of non-methane hydrocarbons (NMHC) Limit values Mass of oxides of nitrogen (NO x) Combined mass of total hydrocarbons and oxides of nitrogen (THC+NO x) L 1 (mg/km) L 2 (mg/km) L 3 (mg/km) L 4 (mg/km) L 2 +L 4 (mg/km) Mass of particulate matter (PM) L 5 (mg/km) (105) (195) 5 5 III 1760<RM (125) (215) 5 5 N (125) (215) 5 5

2 EGR systems are used to reduce oxides of nitrogen (NOx) in combustion and diesel engines by recirculating some exhaust gases back into the air intake system prior to entry into the combustion chamber. The amount of gas injected back into the intake air is controlled by the EGR valve. The injected exhaust gases displace some combustible mixture in a spark-ignition engine or some of the oxygen in the pre-combustion mixture in a diesel engine which reduces the maximum temperature experienced during combustion. Additionally, the exhaust gases more easily absorb heat generated during combustion [2]. This reduction in heat leads to the generation of less NOx compounds that are generated at higher temperatures. In addition, the exhaust gases are higher specific heat capacity than air, and the proportion of water in the returned exhaust gases further reduces combustion temperatures [3]. For spark ignition engines with an exhaust gas recirculation system can reduce NOx emissions by about 20%. Care must be taken to not inject too much exhaust gas or have incomplete mixing of gases as it can lead to ignition failure, increased of hydrocarbons (HC) and carbon monoxide (CO) in spark-ignition engines. In a diesel low-pressure EGR system, the exhaust gasses typically pass through a particulate filter, cooled in a heat exchanger and injected into the incoming air charge in front of the compressor as seen in Fig. 1. The temperature of the EGR and intake air mixture is reduced in a charge air cooler to increase the efficiency of the combustion process. The use of a particulate filter extends the lifetime of EGR system by removing solid particles and preventing deposition and clogging of the system [4][5]. Fig. 1 Low pressure diesel exhaust gas recirculation (EGR) system [5] This type of NOx reduction system is currently the subject of much research and development of its performance and durability. Often the subcomponents of these systems (such as charge air coolers) are manufactured from wrought aluminum alloys to meet weight and cost targets. These aluminum components have to not only withstand challenging mechanical and thermal loads, but they must also have to operate in highly corrosive internal and external environments. Internally, these environments often contain acidic condensates that form with the other by-products of combustion and are contained within recirculated exhaust gases which subject the subcomponents to internal corrosion conditions. The challenges of using aluminum and other metallic materials in these aggressive environments is recognized by the Association of the Automotive Industry (VDA). The VDA has created initial recommendations for test methods to assess the Resistance of metallic materials to condensate corrosion in exhaust gas carrying components [6] These tests combine the use of simulated exhaust gas condensates with newly developed test methodologies to assess the corrosion resistance of metals. However this standard is not yet generally accepted by global automotive manufactures. Each automotive manufacturer and subcomponent supplier has created their own testing methodology to address these concerns. One such approach is an immersion test to verify the corrosion resistance of alloys used in systems containing exhaust gases. These immersion tests often include both immersion and drying cycles. The test samples are then evaluated for the effect of the corrosive media exposure.

3 The future success of these low pressure NO x reduction systems depends on the development of a predictive test methodology which will allow for the rapid and accurate corrosion resistance screening of aluminum alloys for use in this demanding environment. 2. EXPERIMENT Corrosion tests performed on actual engines and automotive systems are typically very expensive and only yield on a relatively small number of sample results in a given time. Laboratory simulation of the operating and environmental conditions seen in these vehicle applications was selected for this investigation. A sample coupon immersion test was developed to enable rapid and cost effective corrosion screening of various materials and corrosion conditions. The test samples were made from two plates brazed together with a corrugated fin material as seen in Fig. 2. Fig. 2 Sample test coupon with two plates containing brazed fins between them (left), samples shown immersed in acid condensate media in temperature chamber (right) The samples were left for 12 hours submerged in the condensate in a closed container in an oven at 50 + / - 5 C (Fig. 2) and after removal were allowed to dry 12 hours at room temperature. The cycle was repeated for 8 weeks. Condensate was checked out after each sample removal if the ph deviation was higher than 0,3 the condensate was replaced by fresh one. The ratio of condensation on exposed areas should correspond to the 50 l/m 2. Table 2 Simulated exhaust gas condensate composition is given below in table. ph 2,6 Nitric acid [mg/l] 50 Chloridic acid [mg/l] 60 Formic acid [mg/l] 70 Sulfuric acid [mg/l] 70 Acetic acid [mg/l] 60 It was decided to construct samples from aluminium alloys known as long life alloys. These are modified EN AW-3017 alloys with better corrosion properties due to the presence of a protective-sacrificial layer (also known as brown band layer) which develops in-situ by metallurgical transformations which occur during the brazing cycle. The materials numbered 5 and 6 are multilayer alloy plates recommended by their manufacturers to be very resistant to corrosion due because of rolled protective interlayer. These composite material structures (multiclad) were fabricated by a core made from long life alloy and from interlayers made

4 from EN AW-3003 or EN AW-1xxx outer layer creates clad. The mechanism of protection of the base material consists the sacrificing "extra"-layers on the exterior of the more noble core creating preferential corrosion in the more active outer layer (corrosion growth will be in a lateral direction) Material compositions tested are listed in Chyba! Chybný odkaz na záložku.. Evaluation of the samples was performed microscopically. Samples were cross sectioned and metallographically prepared for microscopic investigation. Keller solution was used to reveal micro-structural details. Table 3 Composition of used materials Sample Composition (wt.%) Thickness Si Fe Cu Mn Mg Cr Zn Ti Zr Others mm Fin max 0,6 0,7 0,2 1, , ,15 0,07 1. max 0,3 0,2 0,8 1,4 0,02-0,05 0,1-0,15 0,35 2. max 0,1 0,25 0,6 1,2 0,3 0,05 0,05 0,1-0,15 0,3 3. max 0,25 0,6 0,4 1,3 0,1-0, ,49 4. max 0,65 0,44 1 1,5 0,1 0,15-0,1 0,05 0,15 0,4 5.+interlayer 1xxx 6.+interlayer 3xxx max 0,5 0,5 0,5 2 0,03-0,1 0,25-0,5 max 0,3 0,2 0,8 1,4 0,02-0,05 0,1-0,15 0,5 3. CONCLUSION Multiclad samples were found to possess very good corrosion resistance in the immersion testing. The multiclad materials (samples numbers 5, 6) completed the immersion test regime with only moderate surface corrosion attack Fig. 3. Other materials (samples number 1, 2, 3, 4 made from long life alloys) experienced inter-granular corrosion that is not preferred for this type of application. Inter-granular corrosion is not preferred due to its rapid and unpredictable progression rate Fig Etching of the samples revealed that even though all samples corrode in an inter-granularly, this attack was limited by the protective layer of the brown band. It was seen as an opportunity to alter existing pass/fail test evaluation criteria (which does not allow the presence of evidence of inter-granular corrosion) to include a provision to allow some inter-granular corrosion to a certain depth in the brown band layer. These results also reveal the weakest area which was the presence of through-thickness carrion of the fin material.

5 Fig. 3 Microscopic pictures of plates (left) and fins (right) Sample 5 (top), sample 6 (bottom). Magnification 100x. Fig. 4 Sample 1. Corrosion of plate (left), fin (right). Magnification 100x Fig. 5 Sample 2 Corrosion of plate (left) and fin right. Magnification 100x and 50x respectively Fig. 6 Sample 3 Corrosion of plate (left) and fin (right). Magnification 100x

6 Fig. 7 Sample 4 Corrosion of plate (left) and fin (right). Magnification 100xand 50x respectively This static nature of the fluid in this (and other) immersion tests does not simulate the effects of oxygenation of the test fluid during the exposure. This may be an important factor in the resulting corrosion effects observed. Atmospheric gases dissolved in aqueous corrosion medias possess oxygen. Oxygen affects the corrosion rate in these environments due to the rate of oxygen depolarization. Uniform access of oxygen to the surface is also an important factor to consider. Corrosion cells are formed with different aeration and can affect localized attack of the samples, which alters the rate of corrosion reactions Chyba! Nenalezen zdroj odkazů.. Oxygen is not only corrosion stimulator, but it also promotes formation of protective layers and when its content is sufficient in the water can limit corrosion. Follow-up testing will include the effect of oxygenation on the corrosion in this application by introducing air bubbling in the test solutions during the immersion exposure phases. REFERENCES [1] COMMISSION REGULATION (EC) No 706/ laying down, pursuant to Directive 2006/96/EC of the European Parliament and of the Council, administrative provisions for the EC on type approval of motor vehicles with respect to emissions from light passenger and commercial vehicles (Euro 5 and Euro 6) and on access to vehicle repair and maintenance information, [2] BENNETT, Sean. Modern diesel technology: Diesel Engines. Delmar: Cengage Learning, ISBN-13: [3] ERJAVEC, Jack. Automotive Technology: A Systems Approach. New York: Thompson Delmar Learning, 2005, 4th edition. ISBN [4] VAN BASSHUYSEN, Richard; SCHAEFER, Fred. Internal combustion engine. Warrendale: SAE International, ISBN [5] SHAH, K.,Ramesh.; SEKULIĆ, Dušan, P. Fundamentals of heat exchanger design. Hoboken: John Willey sons, ISBN [6] VDA Resistance of metallic materials to condensate corrosion in exhaust gas-carrying components - Testing methods. Bietigheim-Bissingen. DKF, [7] VARGEL, Christian. Corrosion of Aluminium. Kidlington: Elsevier, ISBN