Heatflux corrosion Chloride

Experimental boiler.

When boiler tubes are fouled with boiler sludge (mainly iron oxides) the water in the porous deposit layer is stagnant. Locally wick boiling will occur, often resulting in deposition of silicates, sulphates. Because of the growing solid silicate or sulphate layer the metal temperature will increase locally. Acid boiler water, pa from chloride acid forming seawater salts because of a seawater condenser inleakage, will concentrate in cracks or pores in the solid deposit layer and result in high concentrating ferrous-chloride solutions.

2001 cloride laminated oxide layer
Formation of laminated oxide as a result of chloride enrichment at locally boiling sites at the boiler tube.

Autoclave studies

In total 62 different steel heats were tested in ferrous chloride solutions at 310 °C (10 MPa) in autoclaves during 4 days. After the exposure the morphology of the corrosion scale was studied.
It appeared that at low ferrous chloride concentrations a still well protective adhering magnetite layer was formed. In the high concentrated solutions a laminated non-protective oxide layer was formed. Test solutions of 13 various ferrous chloride concentrations were chosen ranging from 0.005 up to 0.375 molar ferrous chloride.
Amazing is that each of the 62 tested carbon steel heat had a critical concentration above which the oxide layer became non-protective.

Critical ferrouschloride concentration above which the weight loss increases sharply, corresponding with the point where laminated oxide structure is formed.
This critical concentration was considered as the corrosion sensitivity of the steel. The steel samples originated from waterwall tubes of boilers in Dutch electrical power stations. The tubes were cut from the evaporators because of corrosion failures or because of inspection for fouling of the boiler.

Results of critical chloride

By correlating the critical ferrous chloride concentration with the steel composition a regression equation was found with which the critical ferrous chloride concentration for new steels can be calculated.
Critical ferrous chloride = 0.1 Mn + 2 P +0.2 Cr + 0.04 Mo – 0.05
From experience with corrosion in boilers it became clear that in case of severe corrosion in boilers the critical ferrous chloride value was always less than 0.035 molar.

Conclusions

• Aside from water conditioning and heat flux conditions specification for the steel regarding corrosion resistance can be used.
• In case of failures or new boilers it is advised to look after the chemical composition of the steel and to bring that fact in discussion at the boiler manufacturer.
• Boiler steels should have a chemical composition that they can resist a 0.035 molar FeCl2 solution.

See for more information papers nr: 9, 11, 13, 14, 17, 20, 38, 46 and 61.

 

Home » Heatflux » Heatflux corrosion Chloride » Heatflux Corrosion Cloride
2001.jpg
2001.jpg
Test tube exposed under acid boiler water conditions. Seawater condensor leakage was imitated. Corrosion started particularly on the tube half with the highest heat flux and spread out over a large part of the tube. Cross section shows that the deposit layer with the salt deposit is pushed away by the corrosion product magnetite. (foto KEMA, ref 61).
2002.jpg
2002.jpg
Left: Regular laminated oxide in a heavy corroded boiler tube because of a condensor leakage and acid boiler water. Right: Severely blistered corrosion scale. These both types of oxide layer are often found in heavy corroded boiler tubes because of acid forming condensor inleakage. (foto KEMA, ref 8, 23, 61).
2003.jpg
2003.jpg
Corrosion crust with porous and blistered oxide. This type of oxide layer is also often found in heavy corroded boiler tubes because of acid forming condensor inleakage. Foto right shows that a compact oxide layer is present at the metal and at a small distance pores are formed. (foto KEMA, ref 61).
2004.jpg
2004.jpg
A second example of forming of pores behind the compact oxide layer. (foto KEMA, ref 8 and 61).
2005.jpg
2005.jpg
The pores behind the compact oxide on the metal surface develope often in blisters. A second example of blisters developing from pores. (foto KEMA, ref 8 and 61).
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2006.jpg
Near the metal oxide interface the porous and blistered areas have a darker color. Analysis showed that much Chlorid is present in these dark grey coloured areas. (foto KEMA, ref 61).
2008.jpg
2008.jpg
A trypical corrosion scale in acid boiler water. Aside the porosity and the blisters methane enbrittlement can be noticed in the steel. Right: detail of the brittlement. In case of heavy heat flux corrosion much hydrogen (from the corrosion process) can diffuse into the steel. The perlite phase can be transformed by the hydrogen in carbon and methane. This results in internal cracks along the grain boundaries along the perlite phase. (foto KEMA, ref 23, 61).
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2016.jpg
"Characteristic oxide scale on corroded boiler tubes. The 4 stages of the corrosion process can be recognized in the oxide crust. A: Porous layer of oxide deposits, B: Salt layer, C: Top-layer, D: Laminated oxide, E: Steel. (foto KEMA, ref 13)."
2018.jpg
2018.jpg
Example of one of the ca 100 experiments for studying "on-load" or Heat Flux Corrosion. Trend of internal wall temperature, pH-value in water separator and hydrogen production during the last 3 days of experiment 73. (foto KEMA, ref 11).
2019.jpg
2019.jpg
Split test tube 73/180. Thermohydraulic conditions: Steam fraction: 0.09; Mass flow: 1100 kg/m2.s; Heat flux: 360 kW /m2. Dosings into the boiler water: suspension 65 mg/kg; sea water 23 mg/kg. The white lines indicate the locations where samples were removed for microscopic examination. The black lines indicate the irradiated part. (foto KEMA, ref 11).
2020.jpg
2020.jpg
Acid chloride corrosion scale under heat flux conditions. Maybe a stagnant steam bubble caused that underneath corrosion did not occur. (foto KEMA, ref 61).
2021.jpg
2021.jpg
Sections through a corroded tube with 4 stages of "on-load corrosion". · On top of the thin magnetite layer a porous oxide layer (stage 1) is formed in which salt has deposited (stage 2). · Attack of the thin magnetite layer and formation of the top layer (stage 3). · Thick crust of layered oxide under the top layer (stage 4) (foto KEMA, ref 11).
2022.jpg
2022.jpg
Section through samples at locations 105/ 12. On the irradiated side (d) the oxide layer is very porous and there are coarse magnetite crystals. On the not directly heated side the oxide layer is finely laminated. (foto KEMA, ref 11).
2023.jpg
2023.jpg
Section of tube 73/ 180 at locations 117/ 12 (d). At the middle of the side with the greatest heat-input very porous oxide has formed. At the location with less heat-input (end of the oven, d) the oxide is finely laminated. (foto KEMA, ref 11).
2024.jpg
2024.jpg
Sections through samples from tube 61/ 175. Between the oxide and the steel corrosive salt has been deposited (a and b). (foto KEMA, ref 11).
2025.jpg
2025.jpg
Scanning electron microscope pictures of sample 61/ 175/ 103. It is found that the composition of the grey salt and of the clay-like structure is alike. Furthermore, the elements S and Cl are present. (foto KEMA, ref 11).