White Rust Removal and Prevention
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Transcript of White Rust Removal and Prevention
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White Rust Removal &
Prevention
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Removal of White Rust
As noted, white rust corrosion ischaracterized as a localized/pittingtype corrosion and identified by
characteristic white deposits.
However, not all white depositsfound on galvanized steel surface are
due to white rust and not alldeposits, including zinc-rich deposits,will result in localized/pitting
corrosion.
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White Rust Evaluation
Evaluation need to be done todetermine the deposits are better leftalone or if removal is required.
Evaluation can include any or all ofthe following :
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White Rust Evaluation
1. Deposits analysis - determine theinorganic content. It may be thedeposits are calcium based and not zincoxide.
2. Physical inspection of the surface underthe deposits - investigate to determine
if there is pitting corrosion resultingbeneath the deposit. Consider leavingthe deposit alone if pitting is notobserved.
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White Rust Evaluation
3 Age of the equipment and of the deposits- the deposits may be doing more goodthan harm. A tower that is far along inlife expectancy with white rust that hasbeen present for years is probably betteroff left alone.
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Mechanical Cleaning Methods
Virtually all information recommends theremoval of the white rust by brushing withstiff bristle brush (not metallic wire) and
then coating the damaged areas. If thewhite rust build up is light or spotty, itshould be easily brushed off to allow forthe formation of the protective zinc oxide.
This process can be enhanced by theaddition of inorganic phosphate or byreduction of the pH/alkalinity during there-passivation step.
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Chemical Cleaning Methods
In mild cases the area should be brushed (usinga stiff bristle brush) with a mild cleaning solution.Severe cases may require multiple applications ofa more concentrated cleaning solution along with
brushing. Phosphoric acid is an excellent choice, although
other acids such as acetic, glycolic or citric havebeen used with success.
Care should be taken when using these otheracids since they can chelate the base zinccoating.
Overdose may strip the zinc coating from thesteel surface.
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Prevention of White Rust in
New Cooling Tower
White Rust of galvanized steel towercan be a difficult and expensiveproblem.
The start up program for a newgalvanized steel cooling tower should
incorporate the following guidelinesto prevent white rust.
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Prevention of White Rust in
New Cooling Tower
Minimize mechanical damage to he zinccoating especially the flooded interior ofthe tower where aggressive waterchemistry is most likely.
Before tower start up, the tower should bemade free of debris by sweeping or
flushing chemicals of extreme pH (9) as this can initiate zinc corrosion orstrip the coating completely.
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Prevention of White Rust in
New Cooling Tower
Concentrated treatment programchemicals should not be added to areas
where direct contact with tower surfacescan occur. Always add chemicals to anarea of turbulence.
Generally speaking, pH level < 6 or > 9will increase zinc corrosion significantly.
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Prevention of White Rust in
New Cooling Tower
Carbonate and Hydroxide alkalinityare the most corrosive toward zinc.
Acid conditions are to be avoided.
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White Rust Prevention
White Rust may be prevented by severalmethods including :
a) Pretreatment with an inorganic phosphate
passivation program (require minimum of100ppm calcium as CaCO3and 400-450ppmPO4)
b) Operating with the cooling water initially for 45-60 days in the pH range of 7.0-8.0, moderatehardness levels of 100-300 ppm as CaCO3, andalkalinity levels of 100-300 ppm as CaCO3.
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White Rust Prevention
If the cooling tower must be flushedwith acid or caustic type cleaners it isimperative that a bypass piping loop
which isolates the cooling tower beinstalled to prevent damage to thetower.
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White Rust Prevention
Additionally, soft water (300ppm as CaCO3and > pH 9).
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White Rust Prevention
The best practice for successfulcooling tower start up is to operatethe tower for at least 45 to 60 days
with a cooling water of pH between7.0 to 8.0. This should be adequatetime for the protective basic zinc
carbonate layer to form.
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Recommended Cooling Water
Operating Conditions
Parameter Range
pH 7.0-8.0
Chloride as Cl Less than 450 ppm
Sulfate as SO4 Less than 1200 ppm
Alkalinity as CaCO3 50-300 ppm
Hardness as CaCO3 Minimum 50 ppm
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Thank you
