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 Protective hybrid coating of metals exposed to hot corrosion.

Hot corrosion is a process of metal surface destruction caused by gaseous, liquid and solid products of fuel combustion, e.g. in power boilers. These products react (destroy) both with metals and oxide scale. The problem how to reduce the wastage caused by hot corrosion is a current issue in the power industry, chemical and heat generation sector. The reason for the intensive hot corrosion that occurs in power boilers is combustion of low quality fuels and application of combustion technologies that reduce NOx emission. Application of low NOx burners results in formation of reducing atmosphere above the burner level, in particular in the vicinity of boiler walls from the fire side (in the waterwall adjacent layer). Low oxygen hot corrosion usually results in the wastage of the boiler steel at the rate exceeding 0.85 mm/year, while its oxidation rate in air usually does not exceed 0.1 mm/year.

HybridMD coating provides both active chemical and passive mechanical protection of the metal outer surface. It limits the access of corrosive atmosphere to the metal surface and protects it against erosion. It is also resistant in contact with ash deposits. HybridMD coating is resistant in particular to hot corrosion in reducing atmospheres.

The coating shows satisfactory resistance to oxidation up to 500°C. Special applications allow to use it as the anticorrosion protection in humid atmospheres during outages.

HybridMD can be also applied both on new elements and boiler walls during operation.

The advantages of waterwall protection of power boilers against hot corrosion by the application of protective coating on the waterwall surface are as follows:

  • protects against hot corrosion, reduces oxidation rate
  • does not interfere in the combustion process
  • has no impact on the boiler structure
  • does not change the boiler operating conditions
  • reduces the adhesion of deposits
  • increases the resistance to erosion
  • provides the possibility to repair and restore the coating
  • easy to apply
  • cheap, low cost of coating application and its possible repair

HybridMD is a two- or multilayer protective coating, e.g. for the waterwall tubes of power boilers. The base layer contains bonding agents and a special aluminium pigment (layer thickness: 30÷80 µm), the second and next layers contain spherical dispersion and/or flake pigments of aluminium, silicon, or aluminium and silicon compounds. The bond is a mixture of organic resins (layer thickness: 25÷60 µm).

HybridMD can be applied with painting techniques on the shot/sand blasted surface, it requires thermal treatment.

Application of hybrid protective coatings on the boiler waterwalls is a simple operation, absolutely safe for the boiler. It is based on standard methods and equipment used for conventional anticorrosion protection.

The entire process consists of the following steps:

  • blasting of waterwalls surface,
  • application of the first layer with airless sprayer,
  • heating to 300÷350°C,
  • application of top layers with airless sprayer

The process of layers heating is performed by introducing feed water and steam into the boiler cycle, so through a process, which is natural for the boiler. 

The heating/curing technology is performed by the authorized company - Remak-Rozruch S.A., which has an approval of RAFAKO and the Office of Technical Inspection (UDT).

The technology of curing and heating of composite layers was developed by Remak-Rozruch S.A. This technology is protected under patent rights.

The first attempts to apply hybrid coatings on waterwall fragments (inserts) of OP-430e boiler in Siekierki CHP Plant were made in 2001. These coatings are still there. From that time three application series have been made, that result in improvement of the coating and the application method. Those tests were completed with a full protection of the furnace of boiler no. 14 (OP-430). All subsequent tests proved the durability and tightness of coatings. It allows to guarantee their durability for 5 operating seasons without renovation required.

Benefits of coatings:

  • low price,
  • can be applied at manufacturer’s or during operation,
  • possibility to protect inserts and welds,
  • possibility of renovation,
  • easy to inspect,
  • can be optimized for a specific client,
  • long-term protection provided (ca. 5 years)


for scientific and technical issues:
prof. Marek J. Danielewski PhD, Eng. AGH, ul. Mickiewicza 30, 30-059 Kraków
email: daniel@agh.edu.pl,
telephone: (0-12) 617-32-96, 617-24-67,
fax: (0-12) 617-24-93