Principal Research Results

Development of Coating Technology for Preventing Sulfide Corrosion
on Boiler Tubes in Coal Fired Power Plants− Development of a Practical Compound Film of Low-cost
and High Corrosion Resistance−
BackgroundIn coal fired power plants, the concentration of H2S becomes higher in the burner area where there is a strong reducing
atmosphere. Therefore, the boiler tubes are damaged due to sulfide corrosion. The damaged tubes should be repaired by buildup
welding or thermal spraying. If the damage by sulfide corrosion is large, the boiler tubes need to be replaced. Thus, these repairs
become costly. To deal with this problem, we have embarked on development of an economical and straightforward technique of
coating for preventing sulfide corrosion in 2004. We designed the compound film of TiO2 and the carbon film in 2006. The sulfide
corrosion could be reduced by 50%. However, the corrosion resistance of the film did not achieve the required value (10%).

Objectives
This study aims to develop film structure which has high corrosion resistance on the basis of current coating technology.
Moreover, the coating process is simplified.

Principal Results
1. Success in development of compound film that has superior corrosion resistant to 10% or lessIn order to improve the corrosion resistance, the coating film has to improve the gas-tightness. SiO2 is selected as a prime
film material, and SiO2/TiO2/C/TiO2 compound film is developed. As a result of the sulfide corrosion test performed, the developed
film demonstrated corrosion of 10% or less compared to that in the uncoated sample (Fig.1, Fig.2). In addition, this coating didn't
affect the structure of base metal such as carburization.
2. Simplification of coating process for low cost
In previous coating processes, the oxidation or degreasing were performed during each coating as shown in Fig 3(a). In thesimple coating process, the oxidation and degreasing are performed after all coating as shown in Fig. 3(b). The performance of thefilm in simple process is confirmed to be the same as that in previous processes. It is possible to oxidize and degrease by the heat
when the boiler starts. Therefore, the coating process can be simplified. Moreover, the coating material costs are also low as listed inTable 1. Decrease of major repair costs can be expected due to this technology compared with the protection film of 50Ni50Cr flame
spray coating.
Coating of developed film on the inspection of boiler every two years ensures the boiler soundness. It is expected that thetotal cost of boiler life decreases because it doesn't need large-scale repairs such as buildup welding and panel exchange, etc.

Future Developments
In order to prove reliability and applicability of developed film, the durability evaluation will be performed in the coal fired
power plant boiler. After that we will put it to practical use at an early stage.
Main Researcher: Makoto Kawase, Ph. D.,
Research Scientist, Energy Conversion Engineering Sector, Energy Engineering Research Laboratory
Reference
M. Kawase, et al, 2008, “Development of coating technology for preventing sulfide corrosion on boiler tubes in coal fired power plantII”, CRIEPI Report M07016 (in Japanese)1086. Fossil Fuel Power Generation

Test conditions
Speciment metal：
metal： STBA24
Temp：
Temp
： 500oC
Gas： CO2/ H2/ N2/ CO/ H2O/ H2S
Gas：= 12%/ 4%/ 67%/ 8%/ 9%/ 330ppm
Test term：
term： 3016 hours

Iron sulfide (FeS)（Approx.110μ m）

Corrodedlayer

The corrosion layer grows
up to about 190 m.
The corrosion layer

FeS

is hardly observed.

Fe, Cr oxides(Approx. 80μ m）
Coatinglayer
Base metal

Base metal

Non-coating（Increasing weight: 32.0mg/cm2）

Coating（Increasing weight: 1.9mg/cm2）

10μ m

Sand Blast

SiO2 Coating
Heating
TiO2 Coating

Sand Blast

SiO2 Coating
TiO2 Coating
Carbon Coating

Increasing weight ratio

Fig.1 Comparison of sulfide corrosion between developed compound film and non-coating

1.0
Coating on
simple process.

0.80.60.4
0.2
0.0
Non-coating

Heating
Carbon Coating

TiO2 Coating

Fig.2 Performance comparisons of developed films

Heating anddegreasing
under Boiler
Operation

Degreasing
TiO2 Coating

Table 1 Material cost for coating

HeatingFinishFinish(a) Normal Process

Developed film in Improved filmwith SiO2 layer
2006 without
SiO2 layer(b) Simple process

MaterialThickness

SiO2

1.0 μｍCoating
efficiency
0.6

TiO2

0.5 μｍ

0.6789

C40 μｍ

0.75

1272

TiO2

0.5 μｍ

0.6789Total

Fig.3 Coating Process

109

Cost(yen/m2)
1833

4683yen/m26