At first sight the zinc silicate coatings showed promise and answered the film thickness limitation of the silicone coatings by virtue of their ability for sacrificial galvanic protection. Also known as inorganic zinc (IOZ) coatings, they were recognized to provide excellent atmospheric corrosion resistance and were widely used in early years to guard against the potential ravages of CUI. However, the aggressiveness of CUI conditions soon revealed that the sacrificial nature of zinc was also rapidly accelerated, to the extent that their excellent protection was only provided for a duration of a few years. With reference again to NACE RP0198 the document specifically addressed this limitation, stating: “Inorganic Zinc Coatings
or Galvanizing shall not be used under thermal insulation in the 50-150°C (120-300°F) service temperature range for long term or cyclic service. Zinc provides inadequate resistance in closed, sometimes wet environments.”.
Sometimes Inorganic Zinc silicates is crosshatched by narrow lines of corrosion which coincide with joints in the block insulation. This pattern demonstrates where water leaked in at these joints and the zinc coating sacrificed, and eventually
leads to pitting along these narrow lines.
dear Babouic, Asghar
The NACE you refer to, which is a nowadays a “SP” by the way, emphasizes on coating and clearly favours TCA for heavy duty environments. My issue with the NACE SP0198 is that it doesn’t focus enough on the influence of insulation. It’s true; wet saturated insulation creates the unwanted electrolyte, so my point is to engineer a fit-for-purpose insulations systems that allows water/moisture to drain or evaporate. There are several well documented standards which describe the use of drainplugs, aircavities, non-contact insulation or the use wicking materials. Any coating whether it’s zinc, silicate coating or even TSA are subject to the QA/QC regime and the quality level of application. Also the equipment design e.g. chamfer all sharp edges, drain holes in vacuum rings, protrusions etc. have a big influence whether a coating will meet it’s expected lifecycle.
Regards
Johan
Dear Johan and Babouic,
Thanks for share you knowledge. We received one vessel coated with IOZ, vessel to be insulated. Construction wants to additional 2 coats with epoxy to cover IOZ. Please give me your opinion, Can we use Epoxy Coating System (with Inorganic Zinc Primer) under Insulation.
Regards,
Asghar
As SP0198 points out, zinc may also reverse potential at around 60 deg C which would render the carbon steel substrate as the anode. Essentially, the principal consideration is that IOZ is not an immersion service coating, and that's why it is really tank linings that gain favour for intermediate temperature ranges.
Dear Asghar,
I thinks below factors should also brought into consideration:
• Expected life cycle
• Service conditions
• Local conditions like the corrosion class (C1-C5)
• Historical experiences from the asset-owner
• Inspection strategy. RBI? TSA can have longer inspection intervals than IOZ
• Coating application costs
• Selected insulation material.
• Reliability and integrity assessments. What’s the impact on production in case of failure?
When looking at above it’s my experience that there are no good or bad coatings, but it shloud be a balanced consideration between all the factors.
Although the NACE suggest only to use IOZ as a first layer primer as Steve correctly states. I know some parts of the world with a different a quality approach tend to choose otherwise.
Dear Johan,
Please note: Vessel are coated with Inorganic Zinc, to be insulated with 2" Rockwool, Max temp 340 F, Produt type is Hydrocarbon. Construction wants to apply additional 2 coats with epoxy to cover IOZ. Please give me your opinion, Can we use Epoxy Coating System (with Inorganic Zinc Primer) under Insulation.
Based on your above mentioned info I would advise to put as you mention a two component phenolic epoxy novolac over IOZ. Apart from this I would also advise to install drainplugs in the insulation, at the bottom under manholes and vacuumrings (in applicable). And to install Vessel Inspection Plugs in the insulation to enable visual inspections.
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