Passivate or not?

TO PASSIVATE or not to passivate stainless steel cargo tanks was the topic of discussion by Steven Suess of Stork Materials Technology at the National Tank Truck Carriers Cargo Tank Maintenance Seminar.

He pointed out that the development of a continuous, non-porous, insoluble, and self-healing passive oxide film on the tank surface makes stainless steel corrosion-resistant.

Electrochemical testing has shown passivation effectiveness in reducing the general corrosion rate of stainless steel in water. Passive film forms naturally and instantaneously in air and has a thickness of about 1/100 of the diameter of a human hair.

Cleaning required

“The uniformity and effectiveness will be reduced by surface contaminants such as oil, dust, and free iron, so cleaning must be performed prior to passivation,” he said.

Dust and dirt should be washed out with water and/or detergent, using high-pressure water or steam if necessary. Loose iron particles and embedded iron should be treated with 20% nitric acid and rinsed out with clean water.

If there are scratches or heat tint from welding (colors range from light gold to blue to violet to black), the surface should have a fine-grit grind, followed by a pickle with 10% nitric acid and 2% hydrofluoric acid, and then rinsed well.

Stainless steels containing sulfide inclusions require a pickled surface with 10% nitric acid and 2% hydrofluoric acid.

Rust can be removed by immersing the surface in 20% nitric acid and rinsed well, while oil and grease can be removed with solvent or alkaline cleaners.

Residual adhesives respond to solvent cleaner or fine-grit grinding. Paint, chalk, and crayon can be scrubbed with clean water and/or alkaline cleaner.

Rough grind, machining marks, welding arc strike marks, weld spatter, and weld defects can be removed by a fine-grit grind.

Process products such as cutting fluids and oils should be removed with clean water or steam, or dissolved in a suitable solvent. Rouge deposits can be removed by dissolving with moderate-strength nitric acid.

When cleaning is complete, the surface should be smooth and polished with no imperfections or exposed inclusions. The surface will then generally have more corrosion resistance due to a more even passive film layer with fewer locations for corrosion to initiate.

“Rough, ground surfaces and surfaces with imperfections and inclusions have less corrosion resistance,” Suess said, adding that they present non-uniform or discontinuous passive film layers and have areas where salts and moisture collect.

“Passivation solutions for MC300-series stainless steel cargo tanks should include 20% nitric acid by volume at 140° F for 30-60 minutes, followed by a hot-water rinse.”

Ethylenediaminetetraacetic acid (EDTA) and citric acid are more environmentally friendly and can be effective passivating solutions.

Testing procedures

Testing can include a humidity test (95° F, 100% humidity) for 24 hours followed by a visual inspection; swabbing the part with copper sulfate/sulfuric acid solution for six minutes followed by a visual inspection for copper deposits; and salt spray testing for 24 hours.

For the 400-Series stainless steel tanks, the passivation solution should contain 20% volume nitric acid plus 4-6 weight percent sodium dichromate. (The term, weight percent, is often used in lieu of volume percent when referring to solid/granular substances, as opposed to liquid substances. When mixing up a solution, one typically measures in liquids using volume, graduated cylinder, and typically measures in solids/granular substances using weight, analytical balance, or scale.)

The solution should be applied at 110° F to 130° F for 30 minutes, followed by a water rinse.

A 50% nitric acid solution also can be used. It should be applied at 120° F to 140° F for 30 minutes and then followed by a water rinse.

Testing should include humidity (95° F, 100%) for 24 hours followed by a visual inspection.

Certain passivating guidelines should be followed:

  • Chlorides should be avoided with the content in the water not more than 50 parts per million, and the passivation solution should be replaced regularly. Excessive contamination of the solution by iron could render it ineffective.

  • Run control samples and evaluate for effectiveness via laboratory testing.

  • Avoid passivating multiple material types together, especially high-carbon stainless steels.

  • Do not passivate surface-hardened or surface-modified stainless steels.

  • Check the nitric acid concentration regularly. “It is bound to drift over time, and must be maintained within the desire pH range,” Suess said.

  • Refer to ASTM A 380, Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems.

Editor's Note: Suess credited a variety of references for the information he presented, including publications of A H Tuthill, R E Avery, R A Covert, Terry DeBold, J C Tverberg, J R Kearns, and G E Moller.

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