A process for passivation of surfaces of heat exchange apparatus exposed to a synthesis gas containing carbon monoxide, includes the steps of: (i) adding arsine to the synthesis gas to provide a total As concentration in the synthesis gas in range 0.001 to 10 ppmv, (ii) exposing the mixture of hot synthesis gas and arsine to the shell-side surfaces of said heat exchange apparatus to reduce the interaction between the carbon monoxide present in said gas and metals in said surfaces, (iii) recovering a cooled synthesis gas from the shell-side of said apparatus, and (iv) passing the cooled synthesis gas, optionally after further cooling, through a sorbent bed to remove arsenic compounds from the synthesis gas.