Citric Passivation

CITRIC PASSIVATION OF STAINLESS STEEL: A NATURAL, ENVIRONMENTALLY-FRIENDLY PROCESS THAT PREVENTS CORROSION OF STAINLESS STEEL SURFACES

Stainless steel is inherently corrosion resistant due to its high chrome content, however, during subsequent forming and machining operations free iron and other contaminants can be introduced to its surface.  If not effectively removed, these surface contaminants compromise the naturally occurring passive layer of the stainless steel and increase the opportunity for corrosive attack of the base metal.

Citric passivation is a non-electrolytic finishing process that removes free iron and other surface contaminates to maximize the corrosion resistant nature of most stainless steels including all 300/400 series SS and high chromium content steels such as 17-4.

The result is an inert, passive oxide layer rich in chromium oxides for enhanced corrosion protection.  The passivated layer is often superior to the naturally occurring protective oxide layer.

While initially developed for the beverage industry, citric acid passivation is now commonly used in food, medical, aerospace, semiconductor and military applications to name a few.  Our process fully complies with and is certifiable to common industry specifications such as AMS 2700, ASTM A380, and ASTM A967. Periodic testing includes methods such as water immersion, copper sulfate testing, and salt spray (ASTM B 117).

Citric vs. Nitric Passivation Comparison

Nitric acid was long used as the only means to passivate stainless steel before studies showed that citric acid provided a safer, more effective process.  Compared to nitric acid, citric acid is organic, non-hazardous, and safe to use.  The citric passivation process emits no toxic fumes and can be disposed of at a much lower overall cost than nitric waste.

Citric passivation has also been shown to perform equally if not better than both nitric and phosphorous passivation in numerous studies.  A common method of evaluation, electron spectroscopy is used to analyze the ratio of chromium oxide to iron oxide found on passivated surfaces.  The higher the ratio of chromium to iron, the more corrosion resistant the surface.  While the nitric process resulted in an average ratio of 2.1 chrome to iron content, the citric passivation process returned an average of 2.36 chrome to iron content.  While both processes will remove excess free iron and provide superior protection, the citric process is shown to be more effective at maximizing the protective chromium oxide layer.

In another study sponsored by NASA, citric passivation was found to produce higher Fe/Cr oxide ratios than nitric while producing an oxide layer nearly 50% thicker than nitric on 316L coupons.

While most grades of stainless steel can be passivated using nitric acid, there are some exceptions and variations in chemistry and temperature must be tightly controlled to meet industry specifications. There are 8 compositions of the nitric solutions depending on the grade of stainless steel and periodic testing specified. Comparatively, citric acid solutions can passivate the same range of stainless steel grades with a single solution.  Variations in temperature are specified only as a means to control total immersion time.