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Hot Air 30/04/2004 (Corrosion in Exhaust Systems) PDF Print E-mail
Why a mild steal exhaust is not an option?

The majority of customers are aware that stock and free-flow performance exhaust systems are available in MILD STEEL and STAINLESS STEEL and on enquiring about these items, very often a debate arises about the preference of mild steel construction of exhaust systems.


Reasons for mild steel preference

There are claims that: mild steel systems flow better thereby allowing for better performance.

  • Mild steel retains less heat, releasing kinetic energy more efficiently
  • Stainless steel retains more heat, robbing power
  • Stainless steel systems result in higher fuel consumption
  • Mild steel systems are better value for money

BESIDES THE ABOVE FALLACIES, The fact is that MILD STEEL EXHAUST SYSTEMS DON’T LAST VERY LONG.


There are two types of mild steel systems:

  1. Aluminized mild steel
  2. SAE 1010 cold rolled, and hot rolled mild steel

The service life of an aluminized or mild steel system is limited mainly by corrosion. A Maximum life of 1½ years is typical. Internal corrosion has been known to be the principle cause of failure in mild steel exhaust systems.

Automotive exhaust gases are a mix of carbon monoxide, carbon monoxide, sulphur dioxide, phosphorus, lead and other metals, oxides of nitrogen, unburned hydrocarbons and moisture. These gases condense and attack the system internally. The extent of the internal attack can be related to the quantity and temperature of the accumulated condensate in the system. Exhaust vapours are cooled by heat transfer through the pipe walls and condense when temperature falls below the dew point. Unfortunately the condensate has a complex chemical analysis, which does not mix to give a single solution with a single pH value, but rather a solution with separated compounds giving a wide pH range. Where the condensate is in contact with metal, extreme corrosion takes place at a rate mainly dependent upon local metal temperature.

The picture becomes more complicated. Oil carried through the system by the exhaust gases and free carbon, can be deposited on the metal surface and this can impede or accelerate the attack as can design features such as overlap joints, crevices and moisture traps. Vibration and stress levels can also be significant, though, in general they remain low until the mechanical strength of the system is greatly reduced by corrosion. The exhaust vapours above 470° cause negligible direct damage on mild steel, so the front section of the system is relatively unaffected. Although on engine start-up the wall temperature is uniformly below the vapor dew point, the front end of the exhaust system heats up in a matter of seconds and the dew evaporates. Towards the tail end of the system the warming up period is extended and condensate accumulates in the muffler boxes as the result of direct condensation on the walls and packing of the silencer boxes, and condensate trickles down the over-axle pipe. The rear muffler box is particularly susceptible to corrosion. Once condensate begins to accumulate corrosive attack proceeds at a rate, which is exponentially dependant upon temperature.

Exhaust systems tend to last longer the more they are used. It is a fact that a car’s exhaust will last longer if it is regularly heated up for long periods at a time. Under motorway driving conditions, the entire system will dry out in a short period of time. The converse situation is true, i.e. where stop-start or extended idling situations occur, very rapid corrosion takes place.

South Africa is probably unique in its exhaust corrosion problems because of its fuels (alcohol-petrol blends), high and low ambient temperatures, relative humidity, driving styles and corrosive salt spray in coastal regions.

Internal surface temperature

As mentioned, the exhaust temperature varies along the length of the exhaust system.

The manifold section surface temperature can reach extreme temperatures above 800°C. The secondary or down pipe section will run average temperatures of 450 – 500°C. The front silencer box reaches a temperature of around 320°C which is still relatively high.

The centre section of the exhaust will create a problem as the average temperature is between 90 and 104°C. This unfortunately is in the vapour dew point temperature range which is conducive to the most corrosion. The rear section or over axle pipe is well below the vapour due-point temperature and this creates a major corrosion problem. In the rear silencer box the temperatures vary from 60° to 80°C. Once again the temperatures are below the dew point temperature.

Exterior exhaust deterioration

Elements of corrosive salts (coastal regions), moisture and mud contribute to the exhaust systems exterior deterioration. These elements, combined with equipment vibration, uneven road surfaces and varying traffic patterns all contribute to the life (or lack of it) of a silencer system.

Mild steel

This material is very temperature stable and can handle extreme exhaust temperatures with no problems. It corrodes badly in low pH environments when in direct contact with exhaust condensates. The corrosion depth follows a logarithmic curve and once corrosion starts the material deteriorates very rapidly. External corrosion also creates a problem in normal atmospheric conditions. This becomes very severe in high humidity coastal conditions. For mild steel to be used in an exhaust system it must be utilized where the condensate in superheated, such as in the front sections.

To prevent external corrosion the mild steel pipes should be coated with a high quality rust resistant and heat resistant coating.

Where the surface temperatures are below 150°C the use of mild steel should be considered totally unacceptable, unless specifically intended for short duration usage only.

Aluminized Steel

This is essentially mild steel with an alloy coating. This material is subject to corrosion in the form of pitting and crevicing. When condensate settles on this material the aluminized coating actually lifts off the base steel material. Apparently, when pitting, crevicing or lifting takes place the corrosion severity surpasses that of normal mild steel.The aluminized coating is also very unstable at temperatures higher than 250°C. Once this temperature is reached or passed, the coating either lifts or burns off. Weld areas are subject to cracking with deep corrosion attacks. This material is utilized in O.E. exhausts on a number of locally manufactured vehicles and it is not universally used by fitment centres because its price approaches that of 409 grade stainless steel.

Average Motorist VS Motoring Enthusiast

The exhaust replacement industry (most exhaust fitment centers) is geared up to replace failed exhaust units with units available off the shelf, which closely resemble the original stock unit. The old unit is unbolted and removed and a new unit is simply bolted or sleeve-joined into place. Nothing wrong with this procedure. Most of the units utilize thin diameter tubing and reactive type mufflers (causing back pressure), which is good for longevity. The resulting pressure blows accumulated condensate right out the exhaust tail pipe. BUT where the replacement unit is not available off the shelf, the dealer has to decide, with or without the customer’s consent whether to make up a unit employing a universal silencer similar to the stock reactive or reflective silencer, or employing a free-flow (absorption type) silencer.

This is where the corrosion problem arises (generally more condensate accumulates in free-flow silencer boxes). The manufacturer of mild steel silencer boxes is very aware of the economic situation many average motorists are (not) enjoying. The average motorist has become a very astute DIY man and if he could he would replace his own exhaust to save some bucks. The manufacturer has to supply mild steel boxes to the general fitment centre at a price that will allow the fitment centre to compete in a cut throat market where the retail price is a primary factor (in many cases the only factor) and in order to do so, the most cost effective (cheap) materials are sourced for competitive pricing to dealers.

Here is the crunch: A mild steel silencer system, specially one in the free-flow format will very rarely have a service life of 1½ years; in most cases it will just exceed the warranty period! The above applies to the general motorist, PLEASE NOT THE ENTHUSIAST

The enthusiast should by definition be better informed:

  1. A stainless steel system (409) will carry a warranty of at least 5 years (mild steel – 12 months)
  2. It can last up to 10 years.
  3. It does not cost five times more. It costs generally 30 – 40% more than mild steel.
  4. It does no overheat. In fact the inherent heat retention, which is minimal in the 409 grade of stainless generally used, would promote an increase in the exhaust gas velocity (which is a good thing).
  5. In the long run (period of 5 years) the replacement cost of all the failed mild steel units, including the initial mild steel system, would amount to 2½ times the cost of a stainless system.

 

As mentioned before, if money (or the lack of) is a problem, free flow exhausts are available as a complete package, or in separate sections priced separately, and can be purchased as separate sections.

I shall expand on the subject of stainless steel exhaust systems in the near future.

This is all for now. (Thanks Jimmy Georgiou for your contribution)

 
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