Exhaust System Materials

Hannu Jääskeläinen

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Abstract: The most common types of steel used in exhaust systems include ferritic and austenitic stainless steels, as well as various grades of aluminized steels. Exhaust system materials are exposed to a variety of harsh conditions, and must be resistant to such degradation mechanisms as high temperature oxidation, condensate and salt corrosion, elevated temperature mechanical failure, stress corrosion cracking, and intergranular corrosion.

Overview

Materials used for exhaust piping, mufflers, and other exhaust system components—which are discussed in this paper—consist mainly of ferrous alloys. Aluminum alloys are sometimes used as a coating on ferrous alloys to impart additional corrosion resistance. In some cases, nonferrous nickel and titanium alloys are used in exhaust system components in especially demanding and/or high performance applications. Ceramics have also seen limited use in exhaust systems to take advantage of their insulating properties.

Ceramics and specialized metal alloys, albeit with different compositions and properties than those used in piping systems, are also commonly used in substrates for aftertreatment devices—ceramic and metallic catalyst substrates and particulate filter substrates. These materials are discussed in more detail in the papers dealing with aftertreatment.

Ferrous alloys are based on iron-carbon alloys and include carbon steel, alloy steels, stainless steel and cast iron. Alloying elements are added to:

The choice of exhaust system materials is driven by a number of factors including cost, warranty requirements and legislated and customer demands for long service life. As a result, materials used in OEM exhaust systems have changed dramatically and continue to evolve.

Mild carbon steel was the material of choice for exhaust systems for many decades. An iron oxide coating on the exhaust system protected it from atmospheric corrosion to varying degrees. However, it suffered from poor corrosion resistance when exposed to road salt and exhaust condensate. As a result, exhaust systems made from this material had a very short life if exposed to the environment experienced by many on-road vehicles. Applications for carbon steel are currently limited to selected nonroad applications that operate in relatively non-corrosive environments. The corrosion resistance of carbon steel can be greatly improved through the use of a hot dipped aluminum coating. This is often referred to as aluminized steel.

One particularly important ferrous alloy alloying element is chromium. By adding sufficient chromium, stainless steel is formed. When stainless steel is heated, chromium forms a protective chromium oxide coating that delays further oxidation. A minimum of about 10.5% chromium is usually required to passivate the surface and to classify a material as stainless steel. So long as this oxide layer is stable and continuous, the metal substrate is well protected from corrosion. It should be noted that nickel, while used in many grades of stainless steel, is not contained in all grades of stainless steel.

Since about the mid-1990s, plain carbon and low alloy steels have been replaced by stainless steel as the primary material for exhaust systems downstream of the exhaust manifold or turbocharger. This transition has taken place because of market demands for extended warranties, and because of demands mandated by emission standards. Technologies to meet increasingly stringent emission standards can raise exhaust temperatures which makes the task of meeting strength and durability requirements especially challenging. Emission standards also require that exhaust systems are designed in a manner that facilitates leak-free assembly, installation and operation for the full useful life of the vehicle.

From the early part of the 21st century, commodities including many of the alloying elements used in stainless steel, have experienced wide and rapid price fluctuations (see Figure 4 for example). In some cases, costs for these materials have increased by up to ~1000%. In response, stainless steel producers have added an alloy surcharge that can be adjusted as needed to account for these price variations. Many producers have focused considerable research efforts on reducing the sensitivity of the finished product’s price to these alloy surcharges.

Emission control systems such as actively regenerated diesel particulate filters (DPFs) and urea selective catalytic reduction (SCR) have also created new demands on material properties. Active DPF regeneration can produce exhaust temperatures as high as 800°C in parts of the exhaust system that would otherwise operate at much lower temperatures. Also, some commonly used stainless steels, such as type 304, have been found to corrode after exposure to urea decomposition products in high temperature environments.

Common alloying elements found in stainless steel and their effects are summarized in Table 1 [1510][1570].

Table 1
Common Alloying Elements Found in Stainless Steel
ElementEffect
Chromium
  • Essential in forming the passive film. Oxidation resistance increases at Cr levels above 10.5%
  • High Cr content can adversely affect mechanical properties, fabricability and weldability
Nickel
  • Stabilize the austenitic structure to enhance mechanical properties and fabrication characteristics
  • Resistance to stress-corrosion cracking is poorest at approximately 8 to 10% Ni but is restored at levels of about 30% Ni
  • Can minimize spalling of surface oxide during temperature cycling
Manganese
  • In moderate quantities and when nickel is present, performs many of the functions attributed to nickel
  • Interacts with sulfur to form manganese sulfides which can effect corrosion resistance
  • Can improve the adhesion of oxide film and improve oxidation resistance
Molybdenum
  • In combination with chromium is very effective for passive film stabilization in the presence of chlorides
  • Especially effective in increasing resistance to the initiation of pitting and crevice corrosion
Carbon
  • Provides strength in the high-temperature applications
  • Detrimental to corrosion resistance through reaction with chromium to form chromium carbides
  • Detrimental to toughness in ferritic grades
Nitrogen
  • Enhances pitting resistance in austenitic grades
  • Detrimental to the mechanical properties of the ferritic grades
Aluminum
  • Enhance high-temperature oxidation resistance
Niobium and Titanium
  • Stabilizers used to preferentially combine with carbon and nitrogen to reduce the formation of chromium carbides and nitrides. This reduces the possibility of intergranular corrosion.
  • Titanium oxide may adversely affect brazability
Copper
  • Provide corrosion resistance to sulfuric acid
  • Improve formability
Silicon
  • Provides high-temperature oxidation resistance
  • Provides resistance to stress corrosion cracking and to corrosion by oxidizing acids

Of the varieties of stainless steel available, two that are important exhaust system materials are:

Ferrous alloys can contain three important grain structures: ferrite, austenite and martensite. In ferrite, the iron atoms form a body centered cubic (BCC or α-iron) structure with iron atoms at each corner of a cube and one in the center of the cube, Figure 1. Austenite is a face centered cubic (FCC or γ-iron) structure with iron atoms at each corner and on the center of each face of a cube. Interstitial holes in the FCC structure allow austenite to accommodate a greater number of carbon atoms, up to 2.11% by weight, than the BCC structure of ferrite which can accommodate up to 0.0218% carbon. Austenite can be transformed into martensite, a very hard and brittle grain structure that is not normally used in exhaust piping systems.

[schematic]

Figure 1. Structure of Ferrite and Austenite

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