Nickel plating boiler tubes

A. Very difficult to give an opinion. I would say it will depend on the water quality (pH, temperature, type of ions present, etc.) and on the nickel quality, adhesion, integrity (absence of pits or cracks), stability and thickness. You may consider a softer and more stable and ductile thick deposit of electrolytic dull nickel instead of the EN.

A. Depending on how corroded and rough the steel tubes are already, EN will provided good protection to steel in pressurized hot water. The corrosion rate will be low and galvanic corrosion at any pores will be minimal.

However, plating the exchanger in total will be difficult. It will have to be supported so that solutions drain and so hydrogen bubbles can get out. The EN coating will most likely not stick to the SS and will probably flake off. Any bare SS will be cathodic to EN and may or may not accelerate its corrosion.

The zinc anode is only intended to protect the tube ends and tube sheet and does not protect the inside of the tubes.

A. Rather impractical, hence costly, to pretreat and EN plate in situ: Electroclean (lots of bubbles), hydrochloric acid dip, anodic clean in sulfuric acid, Wood's nickel 'strike', electroless nickel plate; with adequate rinsing between steps and DI water before the EN. EN solution is expensive to prepare (about $5/gallon), and temperature control is an issue, although room temperature solutions are available.

The zinc anode will indeed protect firetube exteriors from typical water, e.g., municipal water in a once-through water heater or closed-loop heating water maintained within proper pH and oxygen limits.

For more corrosive conditions, or perhaps for longer life, reduced maintenance and/or cleaner steam, use corrosion-resistant, ASME and ASTM listed boiler tubing, e.g., a Cr-Mo ferritic SS or an austenitic SS per ASTM A213. This would be a better galvanic match with the boiler flanges, etc. Notes: Ferritic steels (plain & SS) have better thermal conductivity than austenitic SS. Generally, thinner-walled SS tubing is permissible cf. plain steel. Have a boiler engineer verify compliance with the ASME Boiler and Pressure Vessel Code.