Corrosion in Exhaust System Vol-1
1.1 Introduction to corrosion in automotive exhaust muffler
The exhaust system of an automotive muffler has lots of prerequisite to accomplish. The foremost one is to reduce the noise level produced by the engine, adequate insertion loss, to access the waste gases out from the rear part of automobile system, to play down the reduction in engine performance, to have a satisfactory service life, back pressure, shape, style, and cost (1). A large variety of exhaust silencers having different design characteristics are available in an automotive market. They can be divided into various types like clamshell mufflers, wrapped mufflers, deep drawn mufflers, lock seam mufflers, welded mufflers, and so forth.
The main reason behind the premature muffler failure is because of the corrosion, fatigue or combination of the both. About 80% of the failure is due to corrosion and rest is by fatigue.
Some of the corrosion mechanisms affecting are as below:
(i) Internal corrosion due to acidic condensate.
(ii) External corrosion due to de-icing agents used on icy roads.
(iii) Material sensitization, especially on the hot spot (material temperatures up to 500º C for rear muffler and 600 0C for front muffler.
(iv)Static loads due to heating and cooling cycles (low cycle fatigue).
(v) Vibrations from the engine (high cycle fatigue) (2).
1.2. Current Status
The material which is consistently used for the exhaust component is basically aluminized mild steel, stainless steel, aluminized stainless steel, ferritic chromium stainless steel, ferritic chromium molybdenum stainless steel. Corrosion is one of the major problems due to which automotive exhaust components have a limited lifetime as it is basically attacking the muffler since long ago. Even Stainless Steel does not possess effective corrosion resistance due to very highly aggressive environment. The life time of a muffler is very less as compared to other parts of the automotive exhaust system. Ferritic stainless steels with a low coefficient of thermal expansion, such as AISI Types 409 and 439 should be preferred but the problem with using the ferritic grades is that they are more difficult to weld, as they are susceptible to grain growth and coarsening at welding temperatures and more prone to pitting corrosion. Aluminised mild steel is having extraordinary property to resist pitting corrosion but not suitable for high temperature corrosion and heat resistant. There by considering all the required properties to withstand the high temperature environment of automotive exhaust silencer powder coated aluminized mild steel can be used. Aluminium-plated or aluminized steel followed by powder coating has been used in the exhaust system of vehicles, for application in high temperature environments.
1.3. Comparison of aluminised powder coating with surface treated aluminised powder coating.
Aluminium-plated or aluminized steel followed by powder coating is not able to resist high temperature salt damage and aqueous corrosion for longer time, but surface crystallized aluminized powder coating exhibits a corrosion resistance higher than that of the existing coating in exhaust gas condensate environment of automotive muffler and shows excellent resistance to oxidation as well with drastically improved service life.
1.4. Objective
The main objective of the work was to develop high temperature heat resistant powder coating for mufflers of automobile applications with enhanced aqueous corrosion, high temperature corrosion and heat resistance.
2.1 Introduction
The exhaust system is split into two categories – the hot end and the cold end. At the hot end of the exhaust system, oxidation and spalling (break away) of the surface oxide layer is the primary corrosion mechanism occurring and at the cold end, pitting corrosion. Typical operating temperatures of the different components are shown in table 1 (1).
Table1 Typical operating temperatures for automobile exhaust components (1).
Categories |
Components |
Operating temperatures(⁰C) |
Hot End Components |
Exhaust Manifold |
800 – 950 |
Front Pipe |
600 – 800 |
|
Catalytic Converter |
600 – 800 |
|
Cold End Components |
Centre Pipe |
300 – 600 |
Muffler |
100 – 300 |
|
Tail Pipe |
100- 300 |
2.2 Corrosion in the hot end of the exhaust system
At the hot end part of automotive exhaust system, material usually subjected to high thermal cycles and high temperature leads to very severe dry corrosion mechanism. Several reactions take place at the high temperature oxidising environment. They may form a protective oxide layer with a thickness following a parabolic law, which persists for some finite time after which spalling occurs and of scaling increases. They may also form a non-protective oxide, which allows rapid oxygen penetration to the metal and subsequent rapid deterioration by internal oxidation, which may render the material brittle and unusable without much observable change of surface or mass. In general, most oxides have different coefficients of thermal expansion to that of the metal from which they are formed, and thermal stresses are set up when the temperature changes. An oxide that forms at high temperatures may therefore lose adherence to the alloy when it is cooled and become non-protective when reheated. Not only does this result in metal loss of the exhaust system, but the flakes can also cause a blockage in the catalyst (1).
2.3 Corrosion in the cold end of the exhaust system.
At the cold part system, condensation of combustion gases produces sulphurous acid, sulphuric acid and low levels of HCl acid, creating critical conditions with acidic pH-values. These condensates, combined with an accumulation of chloride ions and deposits of electrochemically active soot particles, result in a substantial wet corrosive impact on the inner parts of the components. The chloride ions cause a localized breakdown of the hydrated passive film (FeOOH) on the surface of the stainless steel. In the presence of chloride ions, the following reactions take place:
FeOOH + Cl¯ → FeOCl + OH¯ ……………………………reaction (1)
FeOCl + H2O → Fe³+ + Cl¯ + 2OH¯. …………………………..reaction (2)
The overall reaction is:
FeOOH + H2O → Fe³+ + 3OH¯. …………………………..reaction (3)
Fe2+ + 2H2O + 2Cl- → Fe (OH)2 + 2HCl…………………………reaction (4)
The chloride ions thus act as a catalyst, and are not finished. FeOCl approximates the composition of the ‘salt islands’ forming on the passive layer. The passive layer thins out as the outer surface of the film dissolves. At the pitting potential, sufficient chloride has concentrated in salt islands at the surface to start a new anodic reaction at the initiation site. Fresh metal is exposed in areas where the passive layer is depleted. The current density increases drastically and corrosion occurs rapidly which results in the formation of a pit. Once the pit has initiated, it grows auto catalytically. Ferrous (Fe2+) ions are released when the unprotected metal is exposed to the electrolyte. The ferrous ions then attract the negatively charged chloride ions into the initiation site and hydrolysis occurs by the reaction:
Fe2 + + 2H2O + 2Cl- → Fe (OH) ₂ + 2HCl ………………………….reaction (5)
There is a local reduction in pH resulting from the liberation of hydrochloric acid, and this further accelerates anodic dissolution. Chromium strongly passivates the outer surface. It aggravates pitting by hydrolyzing to form a lower pH than that attained by iron (1&2). Exhaust gas contains chloride ions (Cl-), sulphite ion (SO₃²¯), sulphate ion (SOч²¯), carbonate ion CO₃²¯), nitrate ion (NO₃¯), nitrite ion (NO₂¯), formaldehyde (HCHO), formic acid (HCOO¯), ammonium ion(NH₄⁺).
Table 2 Chemical composition of synthetic exhaust gases condensates (mass ppm).
Ion |
Cl¯ |
SO₃²¯ |
SOч²¯ |
CO₃²¯ |
NO₃¯ |
NO₂¯ |
HCHO |
HCOO¯ |
NH₄⁺ |
Activate Carbon |
content |
50 |
250 |
1250 |
2000 |
100 |
20 |
250 |
100 |
1934 |
50g/L |
2.4 Experimental set up to dry hot corrosion
Cold end component test: The complete cycle to which the materials were exposed is shown in Fig.1. Stage I involves the temperature in the salt spray chamber was slowly increased from ambient (±20°C) to ± 50°C while the spray was on. After an hour-and-a-half, the spray was switched off and the specimens remained in the warm, humid, salty environment for another hour-and-a-half. After the vehicle has been driven for a period, it reaches its operating temperature when the exhaust is hot and all water would evaporate — stage II. After the vehicle has stopped, the exhaust starts to cool, and since some condensation containing salt can accumulate on it, corrosion can occur again (stage III). The spray was on for one-and-a-half hours during this period. The specimens were then left overnight at ambient temperature in this humid, salty environment and ten cold-end cycles were run.
Hot end component test: During the driving cycle, the hot end components are exposed to similar conditions to the cold end components, but the temperature they reach is much higher. The simplified driving cycle designed for the hot end components was therefore identical to the cold end component tests, except for stage II. During this stage, the specimens were removed from the salt spray chamber and placed in a high temperature furnace at 900°C for three hours and hence air-cooled. For comparison purposes, an oxidation test was also conducted.
The tests were a modified standard ASTM G54 (5) test and explored the oxidation and spalling resistance of the materials when exposed to a heat cycle. No salt solution was sprayed onto the oxidation test specimen surfaces prior to each cycle. Five cycles of all of the hot end component tests were conducted. Depending upon the temperature and time conditions, a carbo nitriding mechanism as shown on the 304 grade to cross section analysis with SEM (fig.2, 3, 4,). That means urea decomposition on the surface of stainless steel at high temperature could leads to diffusion of interstitial species such as C and N in the metal (3&4).
2.5 Pitting corrosion in the wet section (rear section) of the automotive exhaust systems
Inside the system, condensation of combustion gases produces sulphurous acid, sulphuric acid and low levels of hydrochloric acid, creating critical conditions with acidic pH-values. These condensates, combined with an accumulation of chloride ions and deposits of electrochemically active soot particles, result in a substantial wet corrosive impact on the inner parts of the components.Various stainless steel grades like ferritic, austenitic and manganese containing austenitic steel are used to achieve an optimal combination of properties in the rear section. The resistance of different steel grades to pitting corrosion can generally be compared on the basis of their alloy composition. For pitting corrosion resistance of a material it is important that the dissolution rate is to be low and the material should possess the ability to repassivate quickly during the idle periods [3].
2.6 Requirements of material to be used in automotive exhaust system
The hot front section of the exhaust system (manifold pipes, catalytic converter) requires steels with a high scaling resistance, an ability to resist oscillating stresses due to vibration, optimal elevated-temperature and creep strength, minimum susceptibility to embrittlement and a low coefficient of thermal expansion [4].
In the center section of the exhaust system (center muffler, connecting pipes) resistance to both high temperatures and wet corrosion are needed. Depending on running conditions, either hot conditions prevail (full throttle) or wet corrosion loading dominates (short-distance driving) [4].
In the rear section (rear muffler) wet corrosion becomes the main factor. Inside the system, condensation of combustion gases produces sulphurous acid, sulphuric acid and low levels of hydrochloric acid, creating critical conditions. These condensates, combined with an accumulation of chloride ions, some acidic pH values and deposits of electrochemically active soot particles, can result in substantial wet corrosive loading on the inner surfaces of the components. Compared with this, the external corrosive loads through rainwater, road dust, slush and de-icing salts are almost negligible.[5]