BAT Material Selection & Properties 3 — Questions and Answers
Question 1: Why is dezincification a critical concern with standard brass backflow preventer bodies?
- It increases downstream water hardness
- It causes blue-green staining of connected piping
- It selectively leaches zinc, leaving a weak, porous copper matrix that can fail structurally (Correct answer)
- It accelerates rubber seal degradation through zinc ion release
Correct answer: It selectively leaches zinc, leaving a weak, porous copper matrix that can fail structurally
Dezincification removes zinc from brass alloys, leaving behind a porous, weakened copper structure that cannot maintain pressure integrity, potentially causing catastrophic body failure.
Question 2: Bronze differs metallurgically from brass primarily because bronze:
- Contains aluminum instead of zinc as the alloying element
- Has a significantly higher zinc content than standard brass
- Uses zinc as the primary alloy while brass uses tin
- Is an alloy of copper and tin rather than copper and zinc (Correct answer)
Correct answer: Is an alloy of copper and tin rather than copper and zinc
Bronze is traditionally an alloy of copper and tin, whereas brass is an alloy of copper and zinc — this composition difference gives bronze superior corrosion resistance in many environments.
Question 3: Stainless steel grade 316 is preferred over grade 304 for backflow assembly components exposed to:
- High static pressure above 200 psi
- Temperatures continuously above 200°F
- Chloride-rich environments such as seawater or coastal applications (Correct answer)
- Soft water with very low mineral content
Correct answer: Chloride-rich environments such as seawater or coastal applications
Grade 316 contains molybdenum, which provides significantly better resistance to chloride-induced pitting and crevice corrosion compared to grade 304.
Question 4: Cast iron backflow preventer bodies are MOST vulnerable to corrosion failure when the system water is:
- High in dissolved calcium and magnesium (hard water)
- Maintained at pressures above 125 psi
- Soft, acidic, or aggressive with low pH and dissolved oxygen (Correct answer)
- Flowing at velocities above 8 feet per second
Correct answer: Soft, acidic, or aggressive with low pH and dissolved oxygen
Cast iron corrodes rapidly in soft, acidic, or low-pH water because the protective scale does not form and aggressive water actively attacks the iron surface.
Question 5: Dezincification-resistant (DZR) brass resists dezincification due to small additions of:
- Lead to improve machinability
- Silicon to increase tensile strength
- Arsenic or antimony to inhibit the selective zinc leaching process (Correct answer)
- Nickel to enhance overall corrosion resistance
Correct answer: Arsenic or antimony to inhibit the selective zinc leaching process
Small additions of arsenic (0.02–0.06%) or antimony inhibit the electrochemical process that causes dezincification in brass alloys.
Question 6: Epoxy-lined ductile iron is the preferred body material for large-diameter backflow preventer assemblies installed in:
- Residential potable water service lines
- Small-diameter drip irrigation systems
- Large fire protection mains and industrial water systems (Correct answer)
- Laboratory ultrapure water systems
Correct answer: Large fire protection mains and industrial water systems
Epoxy-lined ductile iron provides the strength needed for large-diameter, high-flow applications in fire protection and industrial systems while protecting the iron from corrosion.
Question 7: When evaluating a bronze backflow preventer body for a swimming pool chemical system, the installer should be MOST concerned about the effect of:
- CPVC fittings connected downstream
- Stainless steel 316 nipples used for installation
- Prolonged contact with bronze body components
- High-concentration chlorine solutions attacking the alloy (Correct answer)
Correct answer: High-concentration chlorine solutions attacking the alloy
High-concentration chlorine solutions can aggressively attack copper-based alloys like bronze, causing accelerated corrosion and potential dezincification of any zinc present.
BAT Material Selection & Properties 3
Why is dezincification a critical concern with standard brass backflow preventer bodies?