Keeping the Lights On

THE MAGAGINE DEVOTED TO NICKEL AND ITS APPLICATIONS October 2003 Volume 19, Number 1

MODERN OVERLAY MACHINES are equipped with real-time digital readouts of major welding parameters, such as voltage, current, travel speed, wire feed rate, torch oscillation, etc., to insure consistent overlay properties and quality.

FREE subscription

PDF of this article (172 kB)

For other recent Nickel Magazine articles on the use of nickel-containing materials in the energy sector, click here.

Nickel alloys reduce downtime in coal-fired power plants faced with severe corrosion challenges. By Gerry Sorell

Nickel magazine, October, 2003 -- A test program, sponsored by the Electric Power Research Institute (EPRI) of Palo Alto, California, U.S.A., has shown that weld overlay technology is a more durable method of protecting waterwall areas in coal-fired power plants than flame-sprayed and diffusion-applied metallic coatings. The tests involved side-by-side exposures of up to 2.5 years in a supercritical boiler fired with a high sulphur/chlorine coal.

Weld overlays of nickel-containing corrosion-resistant alloys were projected to give a service life of at least 8 to 15 years, compared to at best 3 to 5 years for high-chromium coatings.

This finding is important for the operators of coal-fired power plants, many of whom have recently equipped their plants with boilers that are capable of reduced the emission of nitrogen oxides (NOx).

Limiting the amount of air that is introduced into the primary combustion zone of a coal-fired boiler reduces the production of nitrogen oxides from coal-fired power plants. This low-NOx burner technology has been so successful in reducing atmospheric pollution that nearly all coal-fired power plants in the United States have been so equipped.

The technique creates an oxygen-poor reducing environment and lowers the temperature inside the boiler, thus suppressing the formation of NOx. Complete combustion is subsequently achieved by the addition of secondary air higher up in the boiler.

However, the serious drawback to operating in a locally reducing environment is significantly increased corrosion of the waterwalls. These are the boiler tubes and connecting membranes that form a pressure-tight enclosure which contains the combustion gases. The tubes and membranes are typically made of carbon or low-alloy steels. Accelerated corrosion in the lower furnace area is caused by hydrogen sulphide gas and by tube deposits consisting mainly of iron sulphide, both of which originate from the coal.

Weld overlay technology for boilers is not restricted to water-walls, but has been developed also for convection section tubing in superheaters, reheaters and economizers. Metal wastage due to erosion-corrosion can be particularly severe on tubes adjacent to soot blowers. In these high-damage locations, weld overlay cladding is far more effective than mechanically attached tube shields. Full circumfrence weld overlayed tubing is readily fabricated into small-radius coils.

Other coal-derived substances in tube deposits that contribute to corrosion are carbon and chlorides. Metal loss underneath such reactive deposits can reach or exceed 4 millimetres (mm) per year, or roughly ten times the typical corrosion rate in boilers not equipped with low-NOx burners.

The most effective method of protecting waterwall areas is weld overlay with nickel-containing corrosion resistant alloys. Automatic weld overlaying has been recognized by power plant owners/operators to be a cost-effective, long-term solution to severe waterwall wastage.

Automated field overlaying of boiler waterwall tubing was developed in the mid-1980s by Welding Services, Inc. for protecting refuse boilers in waste-to-energy plants against chloride corrosion. To date, the company has overlayed about 20,000 square metres of waterwall panels for refuse boilers, mostly with alloy N06625, which contains about 60% nickel.

With the advent of low-NOx combustion systems, weld overlay technology was adapted for large boilers in coal-fired power plants. Since then, the company has supplied about 20,000 square metres of tube overlay for over 100 utility boilers. The three most widely used overlay materials for this application are S30900 stainless steel and nickel alloys N06625 and N06022.

N06022 is favoured over N06625, based on studies which indicate that N06022 may offer better sulphidation resistance. For boilers burning high-sulphur/chlorine coals or which experience frequent overheating, more corrosion-resistant overlays are available, such as high-chromium nickel alloys N06030, N06052, N06072, N06110 and R20033.

Modern overlay machines are equipped with real-time digital readouts of major welding parameters, such as voltage, current, travel speed, wire feed rate, torch oscillation, etc., to insure consistent overlay properties and quality. Special applications-related measures minimize distortion, produce a favourable (convex) weld bead profile, and assure a continuous high-strength bond. These metallurgical bonds readily withstand stresses arising from differential thermal expansion between S30900 stainless steel weld deposits and ferritic steel tubes. This is not an issue with N06625/N06022 overlays because the thermal expansion characteristics of nickel alloys and carbon steel are quite similar.

Field application of corrosion resistant overlays is accomplished with pulse spray gas metal arc welding, or PSGMAW. Deposition is controlled by means of a preprogrammed weld bead sequence to achieve uniform coverage. Each weld bead is overlapped by the subsequent weld bead to ensure complete, full-thickness coverage. Typical thickness of conventional single-pass overlays is 1.8 mm minimum, and weld metal dilution is generally no higher than 10%. The latest models of overlay machines can cover 0.14 to 0.19 square metres per hour. With 10 machines, an area of 17 to 23 square metres can be covered in a single, 12-hour shift.
 
Gerry Sorell is a consultant to the Nickel Development Institute.

PHOTOS: courtesy of Welding Services Inc.

Electric Power Research Institute (EPRI) Palo Alto, California, U.S.A Web Site: www.epri.com Welding Services, Inc. 2225 Skyland Court Norcross, GA U.S.A. 30071 Tel: 1 770 452 0005 Fax: 1 770 449 4684 Web Site: http://www.weldingservices.com/ GE Welding Specialty Services A Division of GE Energy & Industrial Services, Inc. 1907 Warren Rd. Kansas City, MO U.S.A. 64116 Tel: 1 816 701 4000 Fax: 1 816 701 4004 Web Site: www.gepower.com/businesses/ge_welding/en/index.htm

Free NiDI technical literature: The Role of Chlorine in High Temperature Corrosion in Waste-to-Energy Plants (1998) (NiDI Reprint Series No. 14045) For other recent Nickel Magazine articles on the use of nickel-containing materials in the energy sector, click here.

<< Previous   --   Next >>