Are brass ball valves suitable for direct burial?

No, brass ball valves are not designed for nor are they suitable underground / direct burial applications.

Brass alloy products are not recommended for direct burial or for locations where the environment is highly corrosive, Brass alloys including DZR (Dezincification-Resistant) brass with high zinc content levels are susceptible to accelerated rate of dezincification corrosion, brass alloys with more than 15% zinc content are generally referred to as “Yellow” brass., these alloys have much lower copper content than Red Brass and or Bronze alloys.

CAUTION: Do not confuse yellow brass with the much superior Red Brass and Bronze alloys.

Red Brass, Bronze alloys having less than 15% Zinc content are impervious to dezincification corrosion. These materials are excellent choices for underground / direct burial application. Stainless Steels are also a good choice for direct burial, having no Zinc content they are also impervious to dezincification corrosion.

304 Stainless Steel valves and fittings are composed of approximately 18% chromium and 8% nickel, which gives it excellent corrosion resistance and therefore widely used in residential and commercial applications due to its durability making 304SS a viable choice for underground installations. However, in aggressive soils they too can be adversely affected, therefore 304 SS components must be wrapped with a silicone pipe wrap to protect against accelerated corrosion by poor / acidic soil conditions. Failure to wrap the fitting in any underground application will void the warranty if the failure is soil related.

316 Stainless Steel valves and fittings contain 2-3% of molybdenum in addition to chromium and nickel, which enhances its corrosion resistance, particularly in marine and chemical environments. This makes it suitable for more demanding applications where exposure to corrosive substances is frequent. Molybdenum also improves pitting corrosion of stainless steel. Pitting corrosion occurs in solutions containing halogen ions, most notably chlorides. The higher the salt concentration and temperature, the more serious the attack. Increased chromium and molybdenum improve resistance to pitting. These properties make 316 SS an excellent choice for direct burial where long-term performance in harsh conditions is required.

Solution:

BRONZE Ball Valves with a Curb Box are specially designed for underground shut offs and are highly recommended for applications where a buried shut-off is required.

In Conclusion:

Regardless of the material, it is best practice to install the ball valve inside a valve box that keeps the valve out of the direct soil and protects from water seepage. Understanding the application and how the valve will be buried is essential when selecting the appropriate valve for the system design.

Underground (below grade) piping systems are a special service, which has unique challenges and requires special considerations,

Accessibility should also be taken into consideration.
Direct burying a valve eliminates the possibility to perform maintenance, adjustments, or repair.
Back filling, compacted material impeding valve function.
Bury Depth the valve should be 6” to 12” below the maximum frost line to prevent frees up and frost damage.
Piping loads, as well as ordinary structural loads must be considered.

Additional Resources:

https://www.corrosionpedia.com/definition/384/dezincification#:~:text=Dezincification%20is%20the%20leaching%20of%20zinc%20from%20copper,of%20an%20alloy%20is%20preferentially%20removed%20by%20corrosion.