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Kraft Pulp Digester - Tokoroa
 In
1960 the carbon steel batch digester was overlayered with
309 stainless steel using the submerged arc welding process
but lateral stress cracking occurred on the welded coating
opening a gap for corrosion media to attack the carbon steel
of the digester. In 1984, a Tokoroa company purchased the
first privately-owned Plasma pistol in New Zealand and - following
overseas advice - applied a series of plasma coating samples
to the 1 ft x 1 ft x 3/4" plates. The positive evaluation
of the test plates after one year in service led to the decision
to Plasmaspray approx 800 sq ft to cover the worst affected
areas of the digester.
In April 1985 the Tokoroa contractor was commissioned to
do the job because by this time the digester was losing 1mm
of shell thickness annually and the rate of loss was accelerating.
The designated spray area was vaccuum grit blasted to a controlled
parameter and then sprayed with a nickel-based alloy powder
using a high energy Plasmaspray system. The preparation and
metallising functions were fully automated to ensure a consistent
coating.
At this stage the estimated life of the plasma coating under
such a harsh chemical environment was unknown. Two years was
the customers expectation, but the estimated replacement cost
of $42 million for a new digester justified the very small
cost of the trial coating.
The customers expectations were well satisfied. The coating
lasted six years. A re-coating programme was carried out in
1991 and this lasted until 1997. At the end of the six years
service the plasma spray coating showed signs of chemical
attack, but no substrate thickness loss was apparent in the
coated area.
In 1985 the replacement cost of the digester was $42 million.
Today it is evaluated at more than $100 million. The plasma
spray coating has already prolonged the life of the digester
by 12 years.
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