For years, zinc electroplating has been the standard finish for fasteners, and hexavalent chromium was used over the zinc to protect against corrosion of the parts. With Restriction of Hazardous Substances, also known as RoHS, gaining traction in the United States, many platers are eliminating hexavalent chromium in favor of trivalent, which meets the restrictions for now, but also has some unwanted side effects.
Trivalent chromate is not self-healing, like hexavalent, so handling damage can degrade the corrosion resistance quickly if something is not added to the finish. Often what is added is some form of sealer, which helps with corrosion resistance but will change the friction coefficient of the joint, in many cases making it lower, especially when high hour corrosion resistance is desired.
So, back to the original question in the title: why are my zinc plated screws stretching and breaking? The easy answer is “lower friction”. The question you should be asking is, what is the friction coefficient of my electroplating? If you do not specify, it’s almost certain that it has changed in the last five years, and as a result, you are getting a different clamp load in your bolted joints.
For more information on how changes to your finish can affect your clamp load, contact us at ProvenProductivity@bossard.com.