When it comes to stainless steel tanks, poison pads are actually used for protection, not harm. They are designed to prevent any carbon steel from “poisoning” or contaminating the stainless when they are welded together. (When stainless is contaminated by carbon steel, rust is the result.)
When welding two pieces of steel together, both must become liquid in order to bond. During the liquid state, there will be some areas that the carbon steel “bleeds” into the stainless steel. This reduces the corrosion resistance of the stainless steel. In some thicknesses, the carbon steel contamination does not reach through to the product side of the stainless. In thicknesses of 1/4” or greater, poison pads are generally not required, while 3/16” thick is borderline, and anything less should definitely have a poison pad.
Poison pads and reinforcement pads are similar and may serve the same purpose. Reinforcement pads are added to increase the local strength around nozzles and support structures. Like reinforcement pads, it is always a good practice to include a weep hole in any poison pad.
Poison pads are typically 7GA or 3/16” thick and designed larger than the carbon steel part, protecting the stainless steel from contamination. Many chemical processors choose to have some primed and painted carbon steel components on their stainless tanks, such as the legs and manway covers on the tank pictured. For them, it is an economical choice that doesn’t affect their end product in any way. In some cases, trusses and other structural items are so large that they require beam shapes that are not readily available in stainless steel. Therefore, carbon steel beams or channels are used with stainless steel poison pads providing a barrier.
For small parts it is usually less expensive to provide a stainless steel attachment due to the cost of masking, blasting, and painting. We generally see this with tank leg supports and pipe flanges.
SFI Sales and Engineering personnel are experienced in tank design and fabrication. Often, they are able to review customer requirements and suggest ways, such as these, to reduce costs while maintaining the quality and integrity of the vessel.
Follow this link to learn more: https://www.stainlessfab.com/capabilities-services.html
interested in receiving a quote for a project?Request A Quote