Passivation of EXTERNAL surface of air cooler fin-tubes after cleaning

Q. Dear all,

I'm Joko Lelono, Process Engineer working in a refinery.

We are planning to carry out the chemical cleaning on the external surface of our air fin coolers. The tube materials are constructed from carbon steel while the fins are aluminium, embedded type.

Considering the main dirt is rust /scale on the carbon steel bars and not dust in the fins pitch, we decide to carry out the acid pickling to clean out this scale / rust. We'll use inhibted H3PO4 for pickling, then after rinsing we are going to passivate the tube external surface using NaNO2.

Please advise whether such planning is acceptable or not? I'm reviewing the literature, and so far I can only find that chemical cleaning sequence from degreasing--rinsing--acid pickling--rinsing--passivation is applied only for internal part of tubes / pipes --while for external surface I wonder whether such sequence is still applicable.

Thanks for the responses.