What is Chemical Passivation?
Passivation is the chemical treatment of a stainless steel with a mild oxidant, such as a nitric acid solution, for the purpose of enhancing the spontaneous formation of the protective passive film.
For purposes of restoration of stainless steel surfaces in chemical road tankers and tank containers, the process is better defined in two steps.
The First Step is to remove any free iron or iron compound that is on the surface, otherwise this iron will create a localized site where corrosion and pitting can continue. Acid, such as John-Henry GO-pHER®, is used to dissolve away the iron and its compounds. The surface itself is not affected by this process, except that sometimes the iron removal is so complete that a tank interior may look pristine. It is not, and the second step MUST be performed.
The Second Step is to use an oxidizer, such as John-Henry PASSIVE 8®, to force the conversion of chromium metal on the surface, thus enabling the oxide to form. This will create a chromium oxide protective layer.
Citric Acid vs. Nitric Acid
This acid is sometimes used for passivating stainless steel. At elevated temperatures (~160°F) it can effectively remove iron and its compounds from surfaces. This method is being promoted because citric acid is relatively safer to use than nitric acid, and it produces fewer effluent concerns. While it can do an excellent job of removing iron from surfaces, which is the first step of traditional passivation, it is not an oxidizer and so it cannot oxidize chromium, which is the second step of classic passivation. As it cannot build up the protective layer, this process depends on natural air oxidation – an average two week delay before the tank can go back into service.
Citric acid solutions may be recirculated in a closed loop system for convenience and added economy.
The best chemical method to passivate a stainless steel tank surface is to apply a nitric acid solution. Nitric is a strong mineral acid so it can quickly dissolve iron compounds and other trace metals that are on the surface. It is also a strong oxidizer so it can generate the chromium oxide layer at the same time – a dynamic process that allows for loading a tank in just hours after the procedure.
Nitric acid solutions should not be reused, as their ability to remove and retain iron is quite good. The reuse of the material could leave deposits of iron nitride on the surface – not the desired result.
If free iron present in a stainless steel tank interior, the chromium oxide passive film is probably compromised. The tank’s owner must be apprised of the tank’s condition. This should begin with digital photos and discussion with the owner. Unapproved wipe-downs or spot removals of rust stains should never be performed as a means of finishing up a tank in a manner that is “good enough” to load. A tank wash operator should always give the tank owner the option to restore their tank’s chromium oxide passive film in accordance with the ASTM-A967 standard for passivation. If the decision is made to overlook the tank’s condition, it should be the tank owner’s decision, and not tank wash personnel’s.
The following procedures will assist in restoration:
LIGHT RUST: Passivate. When rust appears to be light or superficial, where a simple wipe would remove the bulk of it, fog the tank with John-Henry PASSIVE 8® solution, per ASTM-A967. See usage rates below.
HEAVY RUST: Wash tank, rinse, and then fog with John-Henry GO-pHER®. See usage rates below.
NOTE: After buffing a tank, a thin polymeric film may remain, preventing the passivation solution from penetrating it and doing its job. (Failure to remove this film will result in a grayish, powdery film remaining on the surface, indicating that the passivation procedure has failed.) Prep tank by using an aqueous solution that is high in solvency, such as John-Henry MAGNUM®. The tank is then rinsed out, followed by passivation with John-Henry PASSIVE 8®.
Light Rust: PASSIVE 8®, Use 1.5 US Gallons/1000 gal tank capacity via Simplicity Fogger. Usually 10 gal, undiluted.
Heavy Rust: GO-pHER®, 5 US Gallons (mixed w/ 5 gal water) via Fogger.
Extra Heavy Rust: GO-pHER®, 5 US Gallons (mixed w/ 5 gal water), 2 events with a rinse after each one, via Fogger.
NOTE: Passivation must always follow the usage of GO-pHER®, regardless of quantity used.
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