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The results of laboratory tests and field

surveys have shown zinc coatings

applied to reactive, high-silicon steels

are more susceptible to brown staining

than zinc coatings applied to low

silicon steels. This is primarily due tothe differences observed in the

structural layout of the zinc coating on

the two types of steel after they have

been galvanized. When low-silicon

steels are galvanized, the eta-layer is

relatively thick and the intermetallic

structure is tightly compact, which make it difficult for any free iron particles to penetrate the zeta -layer and

become oxidized.

However, when high-silicon steels are galvanized, the eta-layer is relatively thin and the intermetallic structure is

not tightly compact. The zinc-iron alloy in the zeta-layer form stall, vertical columns that are spaced in a manner

which allow free iron particles to migrate to the top of the zinc coating. Once these free iron particles reach the

surface of the zinc coating, they may become oxidized and form brown stains on the surface. It is sometimes

difficult to visually distinguish red rust on thebase steel from brown staining. However, acoating thickness gauge

can be used to distinguish between the two since red rust will only form if there is no zinc present at the specific

point on the product where the defect is located. If only brown stains are present, the base steel is not rusting and

the corrosion performance of the galvanized steel is not affected. Since there is the presence of zinc under the

brown stains, brown staining is acceptable according to theASTM A123specification.

Rust Bleeding on Overlapping Surfaces

A customer called and said his newly galvanized steel was rusting, and wanted to know why I didn't

galvanize the whole part. I did galvanize it all, what should I tell him?

Looking at the above pictures the customer sent, it is clear where the rust originated. This condition, called rust

bleeding, is the unsightly dripping of rust (iron oxide) from small gaps created by overlapping or contacting

surfaces on galvanized structures. Cleaning and flux solutions can penetrate gaps less than 3/32", but zinc cannot

due to its higher viscosity. Because zinc cannot enter the surfaces inside the gaps remain un-galvanized.

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The cleaning and flux solutions that remain inside the gaps are vaporized as the steel approaches the galvanizing

temperature of approximately 850 F.

When these solutions are vaporized, anhydrous crystals are left inside the gaps. After the steel has been

galvanized, atmospheric moisture can penetrate the gaps and mix with the anhydrous crystals to form an acidic

and highly corrosive solution. When the solution corrodes the un-galvanized portions inside the gaps, corrosion

products accumulate and leak or bleed from the gaps, leaving a trail of rust down the side of the part.

Now that we can see what it looks like, what is the solution? Proper design of materials to be galvanized is the

best method to prevent rust bleeding, ASTM A385 recommends seal welding all overlapping or contacting

surfaces that have gaps less than 3/32".

Seal welding prevents cleaning and flux solutions from entering the gap. When these solutions are prevented

from entering the gap, rust bleeding will not occur. It is important to insure seal welds are complete and without

imperfectionsor pinholes. Imperfections or pinholes in the seal weld can allow moisture to penetrate the gap and

become trapped inside the sealed area. This creates a safety hazard to galvanizing personnel due to explosive

forces that can develop in the sealed area as the steel reaches the galvanizing temperature. When the enclosed

area is greater than 16 square inches, it is recommended to vent the structure perASTM A385.

The designer and fabricator are responsible for notifying the galvanizer when they plan togalvanize pieces with

overlapping or contacting surfaces.The galvanizer is not responsible for ensuring small gaps have been seal

welded or for repairing damage due to rust bleeding.

You can assure your customer the entire structure was galvanized, and explain rust bleeding from overlapping or

contacting surfaces is easily remedied. The stains can be removed from galvanized steel by rinsing with water

and scrubbing with a nylon brush.

Rust bleeding can be stopped at the source by sealing the overlapping or contacting surfaces. When the electrolyte(in this case, the solution created by cleaning or flux solutions mixing with water) is prevented from entering the

area, the corrosion halts, Sealing can be accomplished with materials such as caulking or epoxy coatings. This

prevents water from penetrating the gaps and mixing with the anhydrous cleaning or flux crystals. These coatings

also prevent rust from exiting the gaps.

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