51314-3844-Corrosion Behaviour of Nitriding Steel Grade Coated with Amorphous A-C:H:Si Coating

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Product Number: 51314-3844-SG

ISBN: 3844 2014 CP

Author: Christian Dipolt

Publication Date: 2014

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Diamond like carbon or a-C:H:Si coatings are well known for their low friction coefficient and high surface hardness also for their high resistivity against corrosion in different solutions hence they are of interest enhancing the corrosion resistance of an non resistant surface like a part made of an nitriding steel grade. A widely range of parts in the area of engineering or marine application are made of steel grades like AISI 4140 or 4340. These steel grades have good mechanical properties after nitriding but facing problems with corrosion and have to be coated. A coating with an amorphous a-C:H:Si layer on top of the compound layer of the nitriding can be a suitable solution for a wide range of corrosion problems of these steels. The nitrided and coated samples of 4140 steel grade have been analysed with potentiodynamic polarisation impedance spectroscopy and salt spraying tests with 5 m% NaCl solution. The analysis of the samples with scanning electron microscope convocal microscop and metallographic investigations showed that the main corrosion mechanism is pitting corrosion starting on coating defects. The investigations showed a dependency of types size and distribution of defects from the surface quality and formation before coating. The corrosions resistance of the samples is comparable to common corrosion resistance steel grades like AISI 304 or even better. Only repassivation of these coatings is not possible yet.