ISO 21452:2025

International
Standard
ISO 21452
First edition
Specification and requirements of
2025-03
thermal spray coatings for power
plant boiler tubes
Spécification et exigences des revêtements de projection
thermiques pour les tubes de chaudière des centrales électriques
Reference number
© ISO 2025
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Recommended thermal spray coating materials for boiler tubes . 2
5 Preparation of thermal spray coatings for variable boiler tubes. 2
5.1 Pre-treatment of variable boiler tubes .2
5.2 Thermal spraying .3
5.3 Post-process of thermal sprayed coatings .3
6 Coating quality and performance evaluation . 4
6.1 Coating quality .4
6.2 Performance evaluation of coatings .4
6.2.1 Microhardness .4
6.2.2 Bonding strength .4
6.2.3 Porosity . .5
6.2.4 Thermal conductivity .5
6.2.5 High-temperature corrosion test .5
6.2.6 High-temperature erosion wear test .7
Annex A (informative) An example of thermogravimetric plot as a function of the number of
hot corrosion cycles .9
Bibliography .11

iii
Foreword
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iv
Introduction
Thermal power generation is the main form of power generation in the world. As an important part of the
thermal power plant, the safe and stable operation of the boiler is very important for the thermal power
plant. However, the four tubes of the boiler (water wall, superheater, reheater and economizer) are in
contact with the flame or high-temperature flue gas for a long time, and the working environment is harsh.
During operation, the tubes are affected by wear, high-temperature oxidative corrosion, high-temperature
operation and other factors, which lead to leakage and directly threaten the safe operation of the thermal
power plant. Thermally sprayed technology is an effective technology to improve the surface quality and
prolong the service life of boiler tubes. So far, thermal spraying technology and metal and metal ceramic
coatings have been widely used in the protection of various boilers (such as circulating fluidized bed boilers
and biomass boilers) in the global energy industry. In the process of power plant operation, the type of
coating is determined by the specific working conditions of various boiler tubes.
This document provides guidance for the application of thermal sprayed coatings on four boiler tubes and
promotes technical progress in the energy industry.

v
International Standard ISO 21452:2025(en)
Specification and requirements of thermal spray coatings for
power plant boiler tubes
1 Scope
This document specifies the procedure and requirements of thermal sprayed coatings for water walls and
superheaters of coal-fired and biomass boilers involving the selection of coating materials, pre-treatment,
12/15CrMoV steel substrate preparation and post-treatment as well as the quality and performance
evaluation of the coatings.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 3233-1, Paints and varnishes — Determination of percentage volume of non-volatile matter — Part 1:
Method using a coated test panel to determine non-volatile matter and to determine dry-film density by the
Archimedes' principle
ISO 11357-4, Plastics — Differential scanning calorimetry (DSC) — Part 4: Determination of specific heat
capacity
ISO 13826, Metallic and other inorganic coatings — Determination of thermal diffusivity of thermally sprayed
ceramic coatings by laser flash method
ISO 14918, Thermal spraying — Qualification testing of thermal sprayers
3 Terms and definitions
For the purpose of this document, the following terms and definitions shall apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
coal-fired boiler
equipment that produces steam or hot water through heat exchange between the heat energy released by
the combustion of coal
3.2
biomass boiler
equipment that produces steam or hot water through heat exchange between the heat energy released by
the combustion of biomass
3.3
water wall
wall mainly used to absorb the radiant heat emitted by the flame and high-temperature flue gas in the furnace
Note 1 to entry: Usually laid on the inner wall of the furnace, the water wall has the role of cooling and protecting the
furnace wall.
3.4
superheater
component of a boiler system that heats the steam above its saturation temperature, also known as a steam
superheater
4 Recommended thermal spray coating materials for boiler tubes
This document recommends the appropriate coatings for the following boiler tubes:
a) water wall of a coal-fired boiler;
b) superheater of a coal-fired boiler;
c) water wall of a biomass boiler;
d) superheater of a biomass boiler.
Generally, damage to tubes includes corrosion, wear, coking and so on. Therefore, on the basis of the failure
type, the coating materials outlined in Table 1 are recommended.
Table 1 — Failures and recommended coating materials for variable boiler tubes
Failures
Recommended
Rollers Tube materials
coating materials
Corrosion Wear Fatigue Coking
NiCrTi
Water wall of a
/ √ √ √ 12/15 Cr1MoV steel NiCr-B C
coal-fired boiler
NiCr-Cr C
3 2
Superheater
FeCrBMoC
of a coal-fired / √ / √ 12Cr1MoV steel
NiCr-Cr C
3 2
boiler
NiCrBSi
Water wall of a
√ √ √ √ 15 Cr1MoV steel
biomass boiler
NiCrMoNb
Superheater of
√ √ / √ 12/15 Cr1MoV steel NiCrMoNb
a biomass boiler
NOTE In this table, “√” means “possessing this failure form” and “/” means “there is no such failure form”.
5 Preparation of thermal spray coatings for variable boiler tubes
5.1 Pre-treatment of variable boiler tubes
The surface of the tubes shall be inspected and reviewed to ensure their suitability before spraying. The
following surface preparation procedure is recommended:
a) Surface cleaning: Remove all kinds of impurities on the surface of the object, such as grease, dirt, oxide
scale and rust, especially the welding part at the root of the pipe. No oxide scale and other impurities
that hinder the coating or coating adhesion are allowed. The suggested steps are as follows:
1) Alkali washing: The boiler tube is cleaned with alkaline solution and the temperature is heated to
70 °C to 80 °C for 48 h. Among them, the mass concentration of Na CO is 0,5 % to 1 % and the mass
2 3
concentration of NaOH is 0,3 % to 0,5 %.
2) After alkali washing, clean the boiler tube with water until the pH value is 7 to 8.
3) Acid pickling: The boiler tube is cleaned with acid solution and the temperature is heated to 70 °C
to 80 °C for 48 h. Among them, the mass concentration of hydrochloric acid is 6 % and the mass
concentration of corrosion inhibitor is 0,3 %.

4) After pickling, rinse boiler tube surface with water to neutral.
b) Surface roughening: Dry sandblasting is recommended. Quartz sand with nominal grain size 1,00 mm
to 1,40 mm or corundum sand shall be used as an abrasive for sandblasting, and 0,83 to 1,00 mm
quartz sand, nominal grain size 0,85 mm to 1,18 mm, is preferred for sandblasting in the furnace. The
compressed air pressure used while sandblasting should be greater than 0,4 MPa and 0,1 MPa for a
suction blast system and a pressure blast system, respectively. Compressed air shall be free of oil and
water.
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