Tackling the Problem of Corrosion
Many machines have to operate in harsh environments. These conditions can lead to problems with corrosion. A simple solution is to use stainless steel fasteners. Stainless steel has an oxide layer on the surface that renders it electrochemically passive. In simple terms, it is much less likely to rust than other steels.
To avoid this sort of corrosion many parts are being made from stainless steel. However, this can lead to problems with another sort of corrosion – galvanic corrosion.
Galvanic corrosion – sometimes called bimetallic corrosion – occurs when two dissimilar metals are immersed in a conductive solution and are electrically connected. One metal (the cathode) is protected, whilst the other (the anode) is corroded. Salt water is the most common conductive solution that causes problems.
Galvanic Corrosion Disasters
In the 17th century, the Royal Navy noticed that the lead sheathing used on their ships caused the rudder irons and bolt heads to disintegrate. Then, when they used iron nails to attach copper sheathing the nails disintegrated. By 1886 when the Statue of Liberty was built the engineers knew to put a layer of shellac between the outer copper skin and the wrought iron frame and this lasted for nearly 100 years until the shellac was replaced with PTFE in the 1980s.
But in 2011 the lighting fixtures in the Big Dig vehicle tunnel in Boston started to fall out of the ceiling. Improper use of aluminium in contact with stainless steel in the presence of salt water had led to rapid corrosion. Replacing the lights cost around $54 million.
Stainless Steel is not always the Answer
Engineered Fastener specialists JET PRESS were approached by a customer to supply a spring clip to hold three stainless steel pipes. The customer specified a stainless steel clip to ensure there were no problems with galvanic corrosion. But there was another problem.
Carbon steel clips are formed in a soft state and then heat treated (Austempered) to give them the desired “spring”. But producing spring clips from stainless steel is very challenging.
Making parts in a temper rolled stainless steel such as ASTM 301 means that the tooling needs to be made to over-bend the parts. This takes them beyond the natural elastic limit and allows for spring-back. This is ok if the part is large enough to allow for this over bend. But when you try to form a “U” gap in a part to fit on a 1mm panel you cannot bend the part enough to allow for spring-back.
So the JET PRESS team had to find an alternative.
Meeting the needs of our customer
The challenge was to make a carbon steel clip that delivered a salt spray rating of 480 hours, even when in contact with stainless steel pipes. JET PRESS developed a Zinc-Nickel electroplate with a trivalent (CrIII) passivate treatment. Then they added clear sealer called McDermid JS500. The combination of these finishes and treatments met the brief. It resulted in a spring steel fastener that, even when in direct contact with stainless steel pipes, was able to pass a 480 hour salt spray test.
Not only were there no signs of red rust, but the new part cost less than a stainless steel version would have.