Ineffective Chemical Cleaning?
Introduction
Ineffective chemical cleaning? Not really… we have encountered situations where engineers and operators complaint about the cleanliness of the boiler as they see a black liquid coming off the blowdown system during first fire.
It´s not difficult to get carried away by a first impression and put into question the effectiveness of the chemical cleaning.
Explanations
This dark watery constituent is in reality a very welcome natural corrosion-protection product called Magnetite and it is formed as the steel surface is exposed to water under certain conditions.
The formation of Magnetite is an electrochemical process controlled by a continuous diffusion of iron ions from the steel surface across the water boundary layer and its stability, morphology and porosity depends on the oxidation-reduction potential, pH, oxygen concentration, pressure, temperature and the influence of the make-up water chemistry (CO2, Cl, Si, Na, Ca, Mg…)
Solubility rises with temperature up to 150ºC, then decreases with a steep drop to 300°C. Pressure promotes the growth of magnetite and thickness Having said that, the reason of the dark appearance of the blowdown during a boiler ramp-up is mainly due to the deposition of low quality magnetite since steady conditions has not been reached yet.
