Why Iridium spark plugs fail early

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Iridium spark plugs - Context

Modern engines rely on more than one spark to ensure an optimum combustion and require smaller spark plugs so there is enough room for all the valves in a cylinder, etc.
This led to the use of a smaller diameter spark plug.
Removing a modern spark plug requires a hex socket of 14mm vs. octagonal sockets of 16mm in the 1980's and 19mm in the 1960's.
As a result and in order to keep a good thermal dissipation throughout the threaded part of the spark plug, constructors increased the length of the screw.
The spark plugs electrodes material has also evolved towards a platinum central electrode and the use of titanium and iridium for making the metal parts more resistant to heat.
Electrically, the spark is now required to be shorter (in time) and with a higher intensity which is often resulting from a higher voltage.
In summary, the spark plug is better built and at the same time exposed to higher thermal and electrical stress.

Iridium spark plugs - Failure #1 - Burnt insulator

The first failure concerns the insulator.
The below pictures show how the insulator suffers extreme heat at the contact with the metal part of the plug.
To the left, a spark plug that was driven for 13,000 Km, to the right 30,000 Km.
One can see how the burns progress.
It is also possible that the inner seal broke letting some combustion gas out.

The only possible preventive remedy when refitting new plugs is to make sure the plug is tightened to 2.5 mKg to ensure optimum heat conductivity with the cylinder head.

13,000 Km

30,000 Km

Iridium spark plugs - Failure #2 - Damaged earth electrode

The platinum center electrode suffers almost no damage compared to the earth electrode which is seen here with a hole at the Km 13,000 only.
The hole increases the gap between electrodes and weakens the spark due to the deterioration of the material inside the worn circle.

Iridium spark plugs - Failure #3 - Completely worn after 3-5,000 Km

Presence of high sulfur content in gasoline combined to frequent uphill driving at an average of 3,000 rpm, resulted in the complete destruction of the iridium spark plugs electrodes after only 3,000 kilometers:

All spark plugs on all four cylinders "melted" uniformly with almost no electrode left on any side:

Iridium spark plugs - Lifetime

If you are experiencing spark plus symptoms, then check your Iridium spark plugs for the above and consider replacing them at an interval of 10,000 Km whatever the manufacturer's recommendation. This is particularly true in countries with gasoline of bad quality (poor octane rating, high sulfur content).

Symptoms of worn spark plugs:

A few misfires (but with no ODBII code generation) upon slight acceleration from speeds between 40 to 60 Km/h.
Fuel consumption increase
Rough idle when engine is cold, although this symptom is common to so many other failures.