9 Myths about Phosphatized and Bonderized Steel
Myth #1: Phosphatized and bonderized (phos/bond) is different from A653 galvanized.
FALSE! ASTM makes no distinction between phos/bond and A653 galvanized with a surface treatment of oil.
Therefore, this type of steel is ordered as “Galvanized Steel Phos Treated,” and is not governed by a separate ASTM spec.
Myth #2: Phosphate treatments can only be zinc.
FALSE! Phosphate treatments (pretreatments that help bond paint to galvanized) can be either zinc or iron, according to Galvinfo.
So what’s the difference?
While zinc can be applied by the steel manufacturer, the coil coater, or by the end-use manufacturer, iron phosphates are mainly used in post-fabrication factory operations.
It’s also easier to apply iron versus zinc and it’s less costly, though it won’t offer the same corrosion resistant benefits as zinc.
People might use iron when the entire surface gets painted to minimize uncoated edges. If cut edges will be exposed, it’s preferable to use zinc. In some cases, iron phos will darken the surface and could cause some tinting issues to the finish coat.
In this article, we’ll talk mainly about zinc phosphate coatings.
Myth #3: Phos-bond is really just a color.
FALSE! The color on the strip is not measured or colored, which means it varies from coil to coil, and order to order.
Because it’s made to be painted, specific tints and colors cannot be requested or produced.
Myth #4: Solvent cleaners are best for cleaning the surface of steel to be phosphatized.
FALSE! In fact, hot alkali cleansing is preferred to get a surface that’s water break-free. “Water break-free” simply means there’s no beading on the surface of the steel when it’s wet.
In the case of solvent cleaning, Galvinfo says it’s very difficult – if not impossible – to remove the oils satisfactorily.
Myth #5: It’s a long process to phosphatize steel.
FALSE! In the world of processing, phosphatizing might be one of the fastest ones.
Though it typically involves a 6-step process (cleaning, water rinse, activator rinse, application of phosphate, hot water rinse, sealing rinse), typical treatment times on coil-line phosphating range between 5 to 10 seconds.
If you’re spray phosphating, it might take a little longer – up to 3 minutes – for the proper weight to develop.
Myth #6: Prepainted steel treated with zinc phosphate has better paint undercutting resistance.
TRUE! Not only is this true, it’s also the main reason you’d choose zinc phosphate over iron. Zinc phosphating also provides excellent bond line durability, according to Galvinfo.
This helps corrosion resistance at sheared edges or damaged areas, so your painted steel won’t start to peel as quickly as with other coatings.
Less peeling means longer lifespans and happier final results.
However, it must be noted that using phos-treated steel for end uses other than painting is not advised. That’s because rust or oxidation is almost guaranteed, and should be expected.
Should the product oxidize (white rust) while in transit, storage or application, your supplier can’t be held responsible for any claims.
Myth #7: The auto industry typically uses galvanneal sheet treated with zinc phosphate.
TRUE! There are a few theories out there for why phosphatized galvanneal steel is the best for corrosion resistance, but what you really need to know is that this is the top of the line when it comes to painting.
Heck, if it’s good enough for that new car you’ve got your eye on, it’s definitely going to be good enough for your building.
Myth #8: Bonderized steel is simply phos-treated galvanized directly off the galvanizing lines.
TRUE! When zinc phosphate steel is produced on the galvanizing line, it takes on a matte grey appearance that we’ve come to call bonderized.
The surface of bonderized steel promotes excellent paint grip and offers superior corrosion protection, making it ideal for steel that’s out in the elements.
Myth #9: You can’t preserve the look of bonderized steel.
FALSE! It’s very common to paint bonderized steel with a clear acrylic coating to preserve its rustic look, though some architectural designs and high-end residential applications still call for phos/bond roofing without acrylic.
While acrylic slows the onset of oxidation, which may be preferable for some uses, others prefer oxidation because it creates a certain patina. For example, you see this patina’d style in many Southwest residential applications.
However, buyers and builders should keep in mind that these end uses offer no product warranty against corrosion.