Because　of　adopting　a　high　air-staging　combustion　ratio　for　Chinese　coal-fired　boilers,　the　water　walls　currently　suffer　from　severe　sulfidic　corrosion　and　slagging.　This　work　aims　to　understand　the　real　deposition　of　ash　particles　and　the　high-temperature　corrosion　behavior　at　fireside,　after　making　detailed　characterizations　for　four　types　of　deposits　or　slags　and　a　corroded　water-wall　tube,　which　were　collected　from　a　real　coal-fired　boiler.　The　profile　section　of　the　corroded　tube　presents　three　layers　with　respective　features.　The　major　phases　for　the　manually　peeled　layer　from　the　corroded　tube　are　identified　as　polymorphous　pyrrhotite　(Fe7S8,　Fe9S10,　Fe1-xS),　galena,　and　lead　oxides.　Above　the　corroded　layer　the　deposits　are　mainly　composed　of　ZnS-　and　Si-Al-rich　phases,　minor　of　Fe-rich　particles.　This　is　mainly　attributed　to　the　direct　condensation　of　gaseous　PbS　or　Pb,　the　sulfidation　of　gaseous　Zn　and　Pb,　and　the　thermophoretic　deposition　of　ZnS-/Si-Al-rich　fine　particles.　The　impact　and　erosion　of　particles　promote　the　splitting　of　the　outer　corrosion　layer　into　small　parts　and　then　peeling　off　from　the　inner　layer.　Additionally,　enrichments　of　various　trace　elements　including　As,　Ge,　Ga,　Th,　and　Sn　are　attributed　to　the　direct　condensation　of　metal　vapors　and　their　revaporization-condensation　from　the　unburnt　particles.　The　oxidation　and　sulfidation　of　iron　are　competitive　and　responsible　for　continuous　corrosion.　Although　Fe　sulfidation　can　be　suppressed　in　the　regions　in　which　either　ZnO　or　PbO　is　stable,　Fe　can　be　sulfided　or　oxidized　in　the　regions　where　ZnS　or　PbS　is　stable.　©　2020　American　Chemical　Society.　All　rights　reserved.