I’d like to examine the corrosion resistance of WC-based coating deposited on a steel substrate by thermal spraying. I have mounted the sample in a cold-curing resin and only let the top surface of coating be uncovered. However, when I immersed this mounted sample in 3.5% NaCl solution for a long time, severe corrosion of the steel substrate always occurred due to the existence of crevice between the edges of coating and resin. This result is not what I expect. Do you have any suggestions for me to do this experiment? Thanks very much in advance!
Dear Dr. Haibin Wang ,
I don't know exactly the size of the sample but, for your free corrosion tests in NaCl solution, you could use cells of the electrochemical type (see attached photo), in polymeric material, of the size you think is most appropriate (we also have one series about 5 times smaller). The sample is placed between the two whitish parts, with an o-ring system, held in place by 4 nuts and bolts positioned symmetrically.
P.S .: to see the whole picture, including the bottom part, please click on it.
Best regards, Pierluigi Traverso
Hi Haibin Wang
An electrochemical droplet cell setup could also be an option. By confining the electrolyte to a droplet, only the area under this would be exposed to electrolyte during a given measurement, meaning you should be able to avoid the corrosion at the interface between the resin and the sample as you are currently seeing. This would also provide you with more localized information, rather than a bulk average.
Dear Pierluigi Traverso,
Thanks so much for your suggestions. I think it is really a good idea. Is this small apparatus designed by yourself or purchased from somewhere?
Best regards,
Haibin
Dear Sam Catarelli,
Thanks so much for your advice. Could you please provide me more details using a schematic diagram in a picture?
Best regards,
Haibin
Dear Haibin Wang
The electrochemical droplet cell is setup like the drawing I've attached. A capillary of some kind (glass, ceramic, polymer...) is used to form the electrolyte droplet and keep it in place. The aperture of the capillary determines how big the droplet is, so how much of the sample you're measuring at one time. The droplet is what makes up your traditional three electrode cell. The counter and reference electrodes are contained within the capillary, which depending on the size of the capillary can be combined in a single electrode. The sample itself acts as the working electrode. The droplet cell can be held static to carry out standard electrochemistry and/or electrochemical impedance measurements, or in some cases it can be scanned over the surface to allow local mapping (Scanning Droplet System/Scanning Droplet Cell Microscopy). The two articles below have some good pictures of real world setups.
Article Numerical modelling of the electrochemical behaviour of 316L...
Article Electrochemical characterization of sub-micro-gram amounts o...
Dear Dr. Haibin Wang ,
the system that you see in the photo has been specifically developed by us, also in different versions (even if conceptually the cell does not change) and produced by our mechanical lab (which unfortunately no longer exists !!!). You can still find similar systems on the market, searching for "cells for electrochemical tests" as keywords. I only tell you two things that I didn't mention in the message: 1) the sealing part of the two parts of the cell, performed with o-rings, defines the area of exposure on your sample; 2) the critical contact area is the sample-o-ring contact zone where crevice corrosion phenomena are possible.
Best regards, Pierluigi Traverso
Dear Sam Catarelli,
I understand now. Thanks very much!
Best regards,
Haibin
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