Stress corrosion cracking (SCC) can slowly destroy your brewery equipment without warning. Your tanks are the core of your brewery, so it is important to understand what SCC is, what causes it, and what to do when your tanks suffer from this type of damage.
What Is SCC
SCC is a form of corrosion that occurs in nearly all alloys, including the grades of stainless steel commonly used in brewery equipment (types 304, 304L, 306, and 306L). As its name suggests, SCC takes the form of a network of cracks that eventually will lead to sudden, catastrophic failure in your tanks.
What Causes SCC
SCC occurs only under very specific conditions. The five conditions are present simultaneously when it occurs:
- Specific corrosive agent or corrodent
- Tensile stress
- Temperatures greater than 125 °F
The most common corrosive agents found in a brewery are chlorides and caustics. Chloride SCC is the bigger threat. Chlorides can be found in the water that comes into your brewery, but it is most troublesome when it comes from the insulation that surrounds your tanks. Most insulation has a very high chloride count and when it gets wet, those chlorides can transfer to the surface of your tanks.
Caustic SCC isn't as common. It is most likely to occur in your clean-in-place system if you store concentrated caustic at high temperatures for an extended period of time.
Tensile stress is created in stainless steel when the metal is worked into shape. Tensile stress is highest where the metal is bent - think of the knuckle of a tank head.
SCC requires temperatures greater than 125 °F. There are plenty of unavoidable high temperature processes in a brewery.
There is no practical way to keep oxygen out of the brewhouse which can be considered a positive for your employees.
The final condition that causes stress corrosion cracking is time. Unless your brewery is generating 1.21 jigawatts of power, you don't have any control over the passage of time in your brewery.
How Common Is SCC
Many brewers will never experience SCC, but it is vital to be aware of the risks and what to do if it does occur, as it can be detrimental to your business.
The most common pieces of equipment affected by SCC are:
- Brew kettles
- Mash mixers
- Hot water tanks
- Liquid adjunct tanks
How to Tell If SCC Is Occurring
There are no early warning signs that SCC is occurring. Therefore, the first signs you will notice are visible cracks or possibly even catastrophic equipment failure.
What to Do When SCC Occurs
If SCC occurs, do not attempt to repair the tank by welding the damaged area. Welding will only make the cracking worse. Unfortunately there is no way to repair SCC once it has taken place. Your best option is to first identify the conditions that lead to the corrosion, then replace the damaged equipment.
If you do not identify and remedy the conditions that led to the damage, your replacement equipment will very likely suffer the same exact fate.
How to Prevent SCC
The only way to prevent SCC from taking place is to eliminate one of the five root conditions. Time, oxygen, heat, and tensile stress are all unavoidable in the brewing process. Therefore, you must eliminate or minimize the harmful corrosive agent or corrodent.
Chloride is by far the most common corrodent and should be controlled whenever possible. The best ways to reduce the amount of chloride in contact with your vessels are to use insulation that is chloride free on your hot tanks and to prevent any water from ever reaching your tank insulation.
You can also choose to use stainless steel that is less prone to SCC such as the following: duplex alloy 2205, alloy AL-6XN, type 444, and carbon-clad steels.
Now that you have a handle on SCC, you should make sure you know the basics of vacuum relief in a brewery. Vacuum failure can be just as devastating but is also easily avoidable as long as you are aware of the situations that cause it.