Cr diffusion coatings on a ferritic-martensitic steel for corrosion protection in KCl-rich biomass co-firing environments

•Lifetime of X20CrMoV12-1 substantially enhanced by Cr and Ni + Cr diffusion coatings•Addition of KCl to sulphate salt accelerates degradation of substrates and coatings•Increased corrosion rates are a result of different synergetic corrosion mechanisms•Chlorides accelerate scale dissolution by active oxidation and chromate formation•Ni increases incubation time but promotes formation of detrimental Ni-S eutectic Three diffusion coatings (Cr, Ni + Cr, Ni-P + Cr) on a ferritic-martensitic X20CrMoV12-1 steel were investigated in a biomass-co-firing atmosphere in comparison to uncoated austenitic DMV 310 N. Exposures were conducted at 650 °C for up to 300 h with pure sulphates as well as a mixture of sulphates and KCl. These aggressive testing conditions were chosen to accelerate degradation of the specimens allowing for an efficient ranking between them. Beneath the deposit of Na2SO4-K2SO4 the coatings proved to be highly resistant whereas addition of KCl drastically accelerated the corrosive attack. This is discussed within the frame of different synergistic high temperature corrosion mechanisms.