Citric acid passivation as an alternative to nitric acid treatments
Citric acid treatments can also be considered as an alternative to nitric acid as both provide the oxidising conditions necessary for passivation.
Citric acid is a less hazardous method and has environmental benefits in terms of ‘NOx’ fume emission and waste acid disposal. Solution strengths of 4-10% citric acid are specified for passivation treatments in ASTM A967.
Passivation is designed to maximise the corrosion resistance of stainless steel parts. It ensures that the products have satisfactory performance and do not fail prematurely. The process is performed after fabrication. Stainless Steels will naturally passivate but in practise they will contain contaminates within the oxide layer such as shop dirt, iron from tooling etc. If not properly removed these contaminates can reduce the effectiveness of the oxide layer and corrosion can begin, rust spots will become evident which will be contaminant particles in the surface of the oxide layer and not the parent material.
In both cases, passivation is needed to maximise the natural corrosion resistance of the stainless steel. It can remove surface contamination, such as particles of iron-containing shop dirt and iron particles from cutting tools that can form rust or act as initiation sites for corrosion. In addition, passivation also can remove sulphides exposed on the surface of free-machining stainless alloys, which also may act as initiation sites for corrosion.
Citric acid passivation has become increasingly popular with fabricators who want to avoid the use of mineral acids or solutions containing sodium dichromate, along with the disposal problems and greater safety concerns associated with their use. Citric acid is considered environmentally friendly in every respect.
Passivation treatment in citric acid baths has been found useful for a large number of stainless steel families, including several individual stainless grades.