After the weld metal cools and contracts, an area of high stress forms and remains inside the metal. In the case of welding, for instance, stress occurs when the metal experiences large variations in temperature following the welding process. Stress may also exist as a residual from the manufacturing process. Whether it’s caused by attached piping or frequent temperature changes (thermal cycling), all pressure vessels experience stress. In most cases, the cracks will stop propagating, but due to the dangers presented by an ammonia leak, it’s important to understand the two conditions that can cause SCC.

These cracks form after the vessel is placed in service, so they’re not detectable during manufacturing. Under the right conditions, microscopic cracks form in the vessel, creating pathways for ammonia release. SCC can happen when the management of oxygen, water, and ammonia in a pressure vessel is overlooked. Above all, it’s important to involve qualified professionals such as your technical supplier, contractor, and professional refrigeration engineer when making decisions around the management of SCC.

If you own, operate, or design pressure vessels, you’ll want to make sure you understand what SCC is, why it’s dangerous, and what you can do to prevent it. Stress corrosion cracking (SCC) is a hazard for any pressure vessel operating in anhydrous ammonia service, and usually occurs near the beginning of a vessel’s life.

Making crack with ammonia video