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What does this to metal roof

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    What does this to metal roof

    My porch roof is r panel. I’ve started noticing these marks. I assume some kind of eggs. What makes them?
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    Looks like fungus.



      From above link:
      Some, those with black marks could be either excess aluminium (Al) or under pickling. (not in acid long enough and some oxides are left of the steel before it goes to zinc).
      Excess Al (> 0.008%) can cause these. Al target is around 0.002-0.004% in the molten Zn.
      But whosever told you about sampling technique has a good point. Al sits near the surface and if your Zn sample sent for analysis was taken deep it will be very different to taken from the surface. I'm amazed sometimes how little thought goes into taking a sample when we can get three decimal points or more of a % accuracy in the analysis, but no thought on how representative that sample was of the whole.

      Your Ni looks low, but not so low to say you were not adding some. Did you add some? If you want Ni then 0.05 - 0.06% target. Its purpose is to modify the reaction between zinc and reactive steel (whose chemistry has awkward Si and P levels). The images with pimply finish could be reactive steel, and there's not enough Ni to stop that.

      You can tell more about these black spots by scratching them to see if they are a deposit on top of zinc (like flux residues) or an actual gap in the coating - a genuine bare patch. If zinc under, then just remove the stain. If no zinc then look to acid pickling being under done, and or Al% being too high.​