Corrosive Effect And Evaluation Model Of Fire Smoke On Metallic Materials

Smoke corrosion as nonthermal damage can cause serious loss to the far field area in industrial fires,due to the materials producing corrosive smoke,diffusion of smoke,short-term effect and acceleration of high humidity.An overall understanding of metal corrosion in smoke atmosphere can help to provide a reference for the determination of smoke damage and a guidance for timely rescue after fire.The work of this thesis are summarized as following:A new experimental system for smoke corrosivity to metals was built,based on the experience and the advantages of previous test protocols,which consists of a tube furnace for smoke generation and an exposure chamber for metal corrosion.The whole system not only keeps the advantage of previous test system giving the greatest differentiation with the most repeatable results and corrosion responses consistent with the expectations based on the known chemistry of the material compositions,but also changes the method way into a direct measurement to metals.6061 aluminium alloy,304 stainless steel and 1010 carbon steel,as three typical metals of equipment,were chosen to be exposed to smoke generated by PVC of 4 g,7 g,10g and 14.5 g at different temperatures ranging from 15 °C to 45 °C and relative humidity from 65%to 95%.The corrosion kinetics was measured by the weight loss experiments.The surface morphology was observed by optical microscopy and scanning electron microscopy.The corrosion products formed on surfaces were identified by energy dispersive spectrometer and X-ray photoelectron spectroscopy.For 6061 aluminium alloy,the corrosion kinetic law under smoke from PVC agrees with a linear rule.At each sampling time,corrosion mass loss of 6061A1 samples increases linearly with PVC quality and temperature and also shows exponential growth with relative humidity.The characteristics of pitting corrosion in smoke becomes increasing obvious with exposure time.Small pits and irregular white corrosion products with microcracks are formed on the metal surface.The main corrosion products of 6061Al exposed to smoke from PVC are aluminum hydroxide and aluminium oxychloride.For 304 stainless steel,the corrosion kinetic law under smoke from PVC obeys the power function,with a reduced development trend of corrosion under a passivating effect.At each sampling time,corrosion mass loss increases exponentially with PVC quality and grows linearly with relative humidity.For the parameter of temperature,mass loss increases linearly with Temperature before 96 hours and exponential with Temperature after then.The characteristics of pitting corrosion for 304SS in smoke becomes increasing obvious with exposure time.Amorphous laminar formations are the most common morphology,with little pits and rust protrusions are found on the metal surface.The formation of magnetite and hydrated ferric oxide gradually cover the surface,slowing down the corrosion rate of 304SS.The main corrosion products are magnetite and hydrated ferric oxide,with little chromium trichloride and ferrous chloride.For 1010 carbon steel,the corrosion kinetic law of 1010CS under smoke from PVC obeys the power function.At each sampling time,corrosion mass loss of CS samples increases linearly with PVC quality and also shows exponential growth with quadratic equation of temperature.For the parameter of relative humidity,mass loss increases exponentially with RH before 72 hours and linearly with RH after then.As humidity increases,the orange pits on the surface become orange lumps with brown powder corrosion products;The rising PVC quality and temperature both lead to the colour of metal substrates and powder corrosion product change from brown to black.High temperature also cause to generate an upper detached corrosion layer.The formation of magnetite and hydrated ferric oxide gradually cover the surface of carbon steel,with morphologies of black flat patch,worm nest,bird's nest and thin laminas with a thorny appearance.The main corrosion products for 1010CS corroded in PVC smoke are magnetite and hydrated ferric oxide,with little ferrous chloride.Corrosion mechanisms were all analysed based on the measurements above.An evaluation model of corrosion degree is proposed,based on the corresponding mass loss threshold of carbon steel when different morphologies and products appear.In order to test the model,an investigation about the corrosive effect of smoke on metals in post-fire disaster environment of an industrial plant was conducted.In the survey at the 4th day after the fire,temperature and humidity and the concentration of ionic contaminants in some areas were measured and metal samples were collected Through analysis by optical and 3D laser microscopes,SEM/EDS and XRD,the corrosion degrees under different concentrations of smoke contamination were compared.The results obtained from the model evaluating corrosion degree of carbon steel in PVC smoke match well with the measurement result in the field survey of the industrial fire.