Corrosion in Cars - Research Paper Example

Corrosion in Cars 482194) Introduction Corrosion is the breakdown of the properties of a material due to its regular interaction with the environment. This causes a serious strain on the auto manufacturer since corrosion resistant materials need to be incorporated into car design, periodic repairs and maintenance of certain components need to be carried out and the value of the car depreciates with time. Research is being carried out into the up gradation of materials by the R&D wing of leading auto manufacturers to see that the problem if not totally eradicated, is mitigated to some extent. ( Shaw Barbara and Kelly Robert, 2006) History of Corrosion Even though corrosion was still prevalent in the 1950’s the main criteria for a car manufacturer was the strength and durability of the body. Cold rolling of low carbon sheets was the accepted practice during those times. Later on, when the shape and features of the automobile underwent changes steels that were more ductile and could be drawn easily was incorporated into the design. Currently various advanced methods of steel fabrication with the use of alternative materials are being followed which are both cheaper and also have a lower weight to volume ratio. ( Hamilton Bruce and Macauley, 1998) Statistics of car life since 1950’ Corrosion was a major criterion in the late 50’s which affected the life of a car. It was around the 60’s that things started to improve and new methods for countering corrosion coming up. a corroded car chassis The graph shows the life expectancy of auto vehicles during the past 50 years and the impact corrosion has had on the life of these vehicles. (Hamilton Bruce and Macauley, 1998 ) Use of Road Salts in colder climates This method of using salt to melt the ice on roads was introduced following World War II which led to an increase of corrosion, affecting the functional and structural integrity of the vehicle. The general appearance of the vehicle was also affected. Some of the common pitfalls of corrosion due the presence of salts were formation of holes on body panels, corrosion of brake linings, wear and tear of the frame and bumper supports, discoloration and pitting effects on surfaces of vehicles. Alternate materials were designed to take care of this type of corrosion; however this led to another form of corrosion called galvanic corrosion which occurs when two dissimilar metals are in contact. In the late 1970’s it was noticed that corrosion was taking place to a large extent on the floor panels, exhaust systems, fuel and brake systems which were less visible from outside. Dust control chemicals like calcium chloride and pollutants like nitrogen oxide and sulphur dioxide combined with rain to form acid rains. This prevented the formation of the natural protective layer formation on vehicle surfaces. (Effects of road salt on Motor Vehicles and infrastructure, n.d) Car Production methods The corrosion due to these severe environmental effects was noticed to be maximum during the mid-1970. Customer dissatisfaction forced leading auto manufacturers to invest more heavily into finding more efficient manufacturing process which combined the use of better corrosion resistant materials. Use of more durable body metals such as stainless steel, aluminium alloys, clad steels, zinc based alloys and galvanized steels were introduced to tide over the problem. Developments in primer and coating technology such as cathodic electro-deposition primer and the use of antichip coatings gained momentum. Use of cathodic electrodeposition facilitated the formation of a layer of uniform thickness with well edged contours. Insulating of body joints and crevices were done using resin sealers. More automation was introduced into the manufacturing process to reduce human errors. The improvements made by the auto industry in combating corrosion were as follows. (Hamilton Bruce and Macauley, 1998) Costs of Corrosion It was estimated by the National Institute of Standards and Technology that in the late 70’s two percent of price of the car was devoted in using rust special paints and protective coatings to overcome the corrosion problem. The various areas in which budgets needed to be reworked were in the use of precoated steels and plastics in the body frame, electro-deposition primers, under body splash shields and in the use of improved sealed electrical systems. In the present context it is estimated that roughly about $250 to $800 per car is being used to overcome corrosion problems. If therefore an average of $500 is used in calculations a total of $7.7 billion dollars is incurred for corrosion protection purposes in the United States per year. (Shaw Barbara and Kelly Robert, 2006) Use of salt in highway de-icing has reduced in most countries although the net savings have not shown any major improvement. Another major factor was the depreciation of vehicles due to this salt related corrosion. It is calculated that a vehicle depreciates by 1 to 2 percent per year in states like New York and Massachusetts that used large quantity of salt. Reasons for Corrosion The formation of an electrolyte on the surface of metal surfaces in a humid environment leads to corrosion with the iron as the anode and atmospheric oxygen as the cathode. Anodic reaction involves dissolution of the metal while the cathodic reaction involves reduction of oxygen. Anode reaction: 2Fe→ 2Fe2+ + 4e- Cathodic Reaction: O2 + 2H2O + 4e- → 4 OH- These can be summed to give a oxidation-reduction reaction as 2Fe + O2+2H2O → 2Fe2+ + 4OH- The corrosion product formed is form of Iron oxide (Fe (OH) 2) commonly called as rust. (Pierrre R Roberge, 1999) Different types of corrosion The different types of corrosion are as follows (i) Uniform corrosion- In this type of corrosion the exposed part which is uncoated undergoes uniform reduction of metal. (ii) Galvanic Corrosion – When two dissimilar metals are in contact in the presence of an electrolyte then by its position in the galvanic series one forms the anode while the other forms the cathode. Aluminium and steel when in contact form a closed circuit with aluminium forming the anode and corroding easily. In such cases spacers or sealers are used to isolate the two surfaces. (iii) Crevice Corrosion- This occurs in areas where small amount of electrolyte is trapped in holes, gaskets and under fasteners. This can be explained as a sort of gradient formation between oxygen at the surface of the electrolyte to the oxygen depleted electrolyte at the bottom of the crack. Dissolution of metal ions progresses and concentration of metal chloride increases. pH value of the system falls below 3 thus setting up an automatic anodic process. Since minor cracks and joints are impossible to eliminate this type of corrosion exists in under vehicle and chassis components. . (Siebert Oliver et al, 2008) (iv) Pitting Corrosion- This is an extension of the crevice corrosion and once a mini crevice has formed propagates rapidly into the interior of the metal and perforates the component. Pits can be formed due to breakage in coating, surface deposits and metal inhomogeneities. (Siebert Oliver et al, 2008) Anode/ Cathode distribution in the corrosion of steel using colour indicators A simple experiment can be used to determine the degree of corrosion on products having surface coating. The nails are placed in test tubes and covered with an indicator solution. The indicator changes from yellow to blue colour indicating the areas where rust happens. This experiment is conducted by taking four samples each of (i) Stainless steel nails (ii) Iron Nails (iii)½ Cu plated nail (iv)½Zn plated nail The Procedure is as follows (a) Each of these items is placed in test tubes containing a gel solution which renders the iron nails motionless. Addition of the Fe2+ and OH- indicators give us an indication of the location of these ions. The gel consists of 1 gm of powdered agar and 2 gm of Nacl. 100cm^3 of deionised water is heated till it just starts boiling. The heating is stopped and agar added till it is dissolved. 2ml of phenolphthalein indictor solution, 1 drop of 6 M HCl solution and 1 ml of 0.01 M potassium cyanoferrate (K3Fe(CN)6) is added and stirred. The solution is then set aside to cool. (b) All the samples are initially cleaned using 0.5 M oxalic acid solution to remove oxide from iron. The agar solution is then poured over these nails which are placed over a Petri dish. (c) Observations are then made after a period of time. The nail that is zinc coated will rust the least as it forms the anode and replenishes the ions that are lost from iron. This is called sacrificial protection. No colouration of the blue tinge occurs. (d) The nail that is coated with copper will show maximum corrosion since the reverse process occurs as iron forms the anode by its position in the galvanic series. Electrons are thus lost by iron thus increasing the blue indication. (e) Ordinary iron nails show different types of colouration at different points. The head shows a red coloration while the shank shows extensive blue colouration indicating the presence of iron oxides. (f) Nails that are coated with stainless steel show no presence of corrosion. Polarisation Curves to determine Pitting Potential Polarisation curves are plotted to measure the pitting potential of SS304L in 3.5% NaCl solution. Stainless steel begins to corrode when exposed to chloride solutions above a particular potential called pitting potential, Ep. During the experimentation process using the potentiodynamic technique the potential at which a current increases suddenly is called Ep. The value of Ep depends on potential scanning rate, surface finish of the sample, amount of air flow into the electrolyte, geometric shape of the sample and immersion of the sample before the test. (Wolynec Stephan and Falleiros Neusa , 2001) The test is performed using with a Princeton Applied Research potentiostat using a scanning rate of 1mV/s which is started 5 minutes after the sample is immersed. In combination when the other factors listed above stainless steel tends to form a passive protective film when exposed to the chloride solution and retards the corrosion process. Therefore a higher potential or Ep value is needed to break this film down in order to form a pit. The graph indicates a pitting potential curve that has been plotted for SS304L m specimens of different surface finishes ranging from S1 to S5 where (i) S1-SS304L with grade 600 silicon carbide paper grinding immediately before immersion into testing solution (ii) S2- SS304L with grade 600 silicon carbide paper grinding followed by immersion in distilled and deionised water for 88hrs. (iii) S3- SS304L with grade 600 silicon carbide paper grinding followed by an exposure to air in a desiccators for 88 hrs. (iv) S4- S4-SS304L with grade 600 silicon carbide paper grinding followed by pickling in 30% H2So4 at 35ºC for 30 minutes, water rinsing and drying. (v) S5- SS304L with grade 600 silicon carbide paper grinding followed by passivation in 20% HNO3 at 35ºC for 60 minutes, water rinsing and drying. Hydrogen Charging Carbon steel metals interacting with Hydrogen sulphide have the tendency to release atomic hydrogen which is readily absorbed into wet steel. The diffusing hydrogen reacts with the microstructures in steel leading to hydrogen induced cracking (HIC), hydrogen embrittlement (HE) and hydrogen blistering (HB). To test the effect hydrogen absorption has in the structural and mechanical properties of steel, specimens were charged with hydrogen and subsequently subjected to tensile tests. ( Wheeler Dean et al, n.d) The charging process involves soaking highly polished test specimens in a electrochemical cell using a platinum anode and the steel specimen as the cathode. A 10% H2So4 solution was taken as the electrolyte added with a small amount of thiourea or arsenic trioxide which prevents the H+ ions from combining to form H2 molecules at the surface of the cathode. 5 seconds after the specimen is immersed a polarization is applied using a 12 volt DC battery as a voltage supply source. A resistance connected in series provides a measure of the charging current. The specimens after hydrogenation are rinsed in distilled water and methyl alcohol before proceeding to tensile testing Comparison of Tensile curves of charged and uncharged specimens The following observations can be noted after observing the curves (i) Figure 1 indicates the stress-strain pattern of a charged and uncharged specimen. Internal hydrogen increased the yield strength by 10% and caused a decrease in elongation by 66%. (ii) The charged specimen did not experience a necking region prior to fracture and the ultimate stress at which this occurred is less compared to the uncharged specimen. Several cracks were also noticed on the specimen indicating its tendency to lose its ductile nature and form brittle. (iii) Figure 3 indicates that the specimen failed by a gradual reduction of cross section area whereas the charged specimens experienced boundary parting causing quasi-cleavage type fracture appearance causing boundaries to slide and separate over each other.( Iskanderani Faisal, n.d) (iv) Properties of charged steel baked to 190ºC for 10 hours and uncharged steels were noted. It is noted that with time the charged specimen experienced an increase in yield strength with the maximum gain in 24 hours. The ultimate strength on the other hand experienced a sharp drop falling from 575mPa to 530 mPa. The UTS vs ageing time at 190ºC for hydrogenated(H) and un-hydrogenated (UH) steel. (Wolynec Stephan and Falleiros Neusa, 2001) Steps taken to reduce corrosion The methods involved in mitigating corrosion essentially consist of 2 factors. (a) Use of better corrosion resistant alloys in vehicle manufacture. Magnesium based alloys like AZ80 and ZK 60 have found many takers in the automobile industry. This has lower weight to volume ratio and is more corrosion resistant. (M. Hilpert and L. Wagner, 1999) (b) Use of Protective coating on surfaces like metallic coating, Inorganic coatings and application of organic coatings help in mitigating surface exposure to corrosive environments. (c) Metallic coatings of chromium and cadmium can be deposited by electroplating, spraying and hot dipping. (Designing against Corrosion, 2000) (d) Providing sacrificial anodes like zinc also help in reducing corrosion. By its position in the electrochemical series zinc forms the anode and corrodes earlier than iron thereby protecting it.( Guide to Corrosion Protection, 1999) (e) Corrosion inhibitors also hinder corrosion progress. These are classified as inorganic inhibitors like sodium chromate, organic anionic like mercaptobenzotriazole and organic cationic having wax like solids. Current Practices and Future trends The current practice of using new and improved coatings and materials like GFRP and CFRP has gained momentum. Both glass reinforced plastic and fibre reinforced posses excellent qualities that could be used in auto manufacture like low density, high strength, high stiffness to weight ratio, excellent durability and great flexibility in design. Other properties include excellent environmental resistance and the ability to bond easily with dissimilar materials. (Wang Weimin and Li Qingfen , 2001) Conclusion The future research into corrosion is into finding more efficient alloys for building cars and also coating the surfaces more effectively to impede corrosion. Chromate though used as an inhibitor is highly carcinogenic. Figuring out how long a particular material will last and ways to predict its life with respect to corrosion also form an important part of the studies undertaken to investigate corrosion Reference Lists 1. “Designing against Corrosion”, Automotive Steel Design Manual, April 2000, pp. 3.7.1-3.7.11 2. “Effects of road salt on Motor Vehicles and infrastructure”, Highway Deicing, n.d, p.p 31-36 3. “Guide to Corrosion Protection”, Auto steel partnership, Light Truck Frame Project Team, 1999, p.p 3.1-3.2 4. “Hydrogen degradation of high-strength steels”, Journal of Achievements in Materilas and Manufacturing Engineering Volume 37, Dec 2009, p.p 193-197 5. Hamilton Bruce and Macauley, “Competition and Car Longevity”, Resources for the Future, March 1998, p.p 2-3 6. Iskanderani Faisal, “The Resistance of a Control Rolled and Aged Steel to hydrogen Induced Cracking (HIC), Chemical and materials Engineering Department 7. Pierrre R Roberge, Handbook of corrosion engineering, 1999, p.62, Mc Graw-Hill 8. Shaw Barbara and Kelly Robert,” What is Corrosion”, Electrochemical Society Interface, 2006, p.25 9. Siebert Oliver et al, Perry’s Chemical Engineers Handbook, 2008, p.25-4, Mc Graw Hill 10. Wheeler Dean et al, “Hydrogen effects on strain Induced Martensite formationtype 304L stainless steel, US department of energy, n.d, p.4 11. Wang Weimin and Li Qingfen, “An expert system in FRP Composite Design”, J.Mater.Sci.Technology Volume 17 No 5, 2001, p.537 12. Wolynec Stephan and Falleiros Neusa, “Correlation between Corrosion Potential for AISI 304L austenitic Stainless Steel in 3.5 % Nacl Aq.solution, December 2001, p.78. 13. M. Hilpert and L. Wagner, Corrosion fatigue Behaviour of the High-strength Magnesium alloy AZ80, Journal of Materials Engineering and Performance, 1999, p.402 Read More