Research On Salt Spray Corrosion Mechanism And Performance Evaluation Method Of Solar Absorbing Coatings

The flat-plate solar collertors have achieved a dramatical development,expecially the vacuum deposited solar selective absorbing coatings become a well-respected star products due to the advantages of high absorptance,low emittance,simple structure,safe and reliable operation,easy to maintain and integration of building.But,our country has a complicated climit that the south coast has high air humidity and salt contentin and the north has saline-alkali land and heavy smog.So,in practical applications,we found that the conponents and microstructure of flat-plate solar collectors will suffer from various degradations because of the salt spray,and the surface of the coatings will change to darker,which will lead to the performance degradation of the coatings.However,there is not an effective evaluation method to assess the corrosion resistance of the coatings in both home and abraod.Therefore,to investigate the evaluation method of salt spary corrosion for solar absorber coatings is of great necessary.In this dissertation,with the help of artificial simulated neutral salt spray accelerated corrosion environment,we investigated the influence of salt spray concentration,corrosion temperatures,and corrosion time on the performance of the solar absorber coatings,and also analysed the corrosion mechanisms and the corrosion rules.In the final,we come up with an evaluation method of the salt spray corrosion.In this experiment,we have obtain the following important conclusions:(1)With the increase of the corrosion time,corrosion temperature and the corrosion concentration of the NaCl salt spray,the surface of the coatings will generate some micro-holes and micro-cracks;and the amorphous will gradually disappear and left some big grains;and the interfaces of the sublayers become wider and hard to distinguish.The elements in the coatings also experience a noticeable diffusion,expecially,the oxygen content on the surface of the aluminum substrate and antireflection layer has slightly increased.The microanalysis indicates that plenty of CrOx,Al2O3,SiOx and a handful of AlCr2,AlCr3,AIN and Cr3Si have emerged in the salt spray corroded coatings.(2)The salt spray corrosion mechanism of the solar absorber coatings is that the NaCl salt spray deposites on the surface of the coatings and forms a thin electrolyte membrane.The ions from the electrolyte membrane will diffuse along with the grain boundaries or some other defects and reach the absorbing layer and the aluminum substrate,therefore the electrochemical reaction takes place.Expecially the Cl ions will react with the Cr,Al,CrOx,CrNy,CrOxNy and Al2O3,transfering them to corresponding chloride and hydrogen chloride.Afte drying in air,the chloride and hydrogen chloride will finally form Cr2O3 and Al2O3.(3)We have obtained the relationship between optical properties and corrosion time,which can be describe by an experience equation a = ?0 +A1t1+A2t2+A3t3 and the emittance(?)changes with the corrosion time(t)can be described by the ?=?0 +B1t+ B2t2 + B3t3.(4)The equation of and the above exprical quatiions are supposed to describe the change of the optical properties of the Cr-O-N series solar absorbing coatings.