Materials: Concrete, Corrosion & Polymers

Quick Answer: Environmental engineers pick materials that survive chemical attack, moisture, and mechanical loading. Focus on corrosion mechanisms (galvanic, pitting, MIC), concrete sewer corrosion, and polymer compatibility for pipes and liners.

Treatment plants, conveyance systems, and containment structures operate in aggressive environments. Selecting the wrong metal or liner leads to leaks, contamination, and costly replacement — material science on the FE Environmental exam (~5%) tests whether you understand why materials fail and how to prevent it.

Atomic Structure and Bonding (Applied View)

Materials behave according to bonding type:

You do not need quantum mechanics — know that metals donate electrons in corrosion cells and polymers are long-chain molecules whose side groups determine chemical resistance.

Corrosion Fundamentals

Corrosion is an electrochemical process requiring anode, cathode, electrolyte, and metallic path.

At the anode (oxidation): ( \text{Fe} \rightarrow \text{Fe}^{2+} + 2e^- )

At the cathode (reduction, in aerated water): ( \text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^- )

Galvanic corrosion occurs when two dissimilar metals contact in electrolyte. The more active metal (lower nobility) corrodes preferentially. Example: iron connected to copper in saline water — iron is the anode.

Pitting breaks localized passive films on stainless steel in chloride-rich environments (coastal, de-icing salt).

Microbiologically influenced corrosion (MIC) — sulfate-reducing bacteria generate H₂S and acidic microenvironments under biofilms in oil pipelines and buried ductile iron.

Corrosion Control Strategies

1. Material upgrade — 316 stainless vs. 304 in chlorides; HDPE instead of unlined steel.
2. Cathodic protection — sacrificial zinc/magnesium anodes or impressed current.
3. Coatings and linings — epoxy, polyurethane, cement mortar lining in ductile iron water pipe.
4. Chemistry control — pH adjustment, dissolved oxygen removal, corrosion inhibitors (orthophosphate in drinking water).
5. Design — eliminate crevices, isolate dissimilar metals with insulators, avoid stagnant zones.

Worked example — galvanic series concept: A bolt of plain carbon steel attaches a copper grounding strap in a wet sump. The steel bolt corrodes rapidly because it is anodic to copper. Replace with stainless hardware or isolate with nylon washers.

Concrete in Environmental Structures

Concrete is Portland cement + aggregate + water. Reinforced concrete appears in clarifiers, aeration basins, and headworks.

Ordinary deterioration: Carbonation lowers pH at rebar surface, depassivating steel and causing rust expansion and spalling.

Sewer corrosion (biogenic): Sulfate in wastewater is reduced to H₂S in anaerobic zones. H₂S gas escapes to the headspace; bacteria on moist pipe crown oxidize it to sulfuric acid, attacking concrete above the flow line.

Mitigation:

• Increase turbulence/aeration to limit H₂S release
• Ventilate and scrub headspace gas
• Apply epoxy or calcium aluminate linings
• Use PVC or HDPE in new construction in known corrosive sewers

Polymers and Thermoplastics

Check temperature — PVC softens above ~60°C. UV degradation affects exposed plastics; bury or coat.

NSF/ANSI 61 certification is required for materials in contact with potable water in the U.S.

Mechanical Behavior

Stress (\sigma = F/A) — tension, compression, shear in supports and soil-loaded structures.

Strain (\varepsilon = \Delta L/L_0) — elastic recovery if below yield.

Young's modulus (E) — stiffness; steel E ≈ 29×10⁶ psi; concrete Ec ≈ 3–5×10⁶ psi depending on mix.

Fatigue — cyclic loading from pumps and mixers causes crack growth below yield strength.

Creep — slow deformation under sustained load in polymers and concrete at stress and temperature.

Environmental FE items may ask you to compare allowable stress to applied stress for a simple member or identify which material is appropriate for a given chemical exposure table.

Phase Diagrams and Alloys (Light Coverage)

Iron-carbon phase diagrams explain steel grades. Stainless steels contain ≥10.5% chromium forming a passive chromium oxide layer. Ductile iron offers higher toughness than cast iron for water mains.

Failure Analysis Checklist

When a scenario describes premature failure, ask:

• Is the electrolyte present (water, soil moisture)?
• Are dissimilar metals coupled?
• Is chloride or H₂S present?
• Is the material stressed while corroding?
• Was the wrong polymer specified for solvent or temperature?

Exam trap: Stainless steel is not universally corrosion-proof. Chlorides and lack of oxygen in crevices destroy passivity — specify correctly or use non-metallic alternatives.

Material science supports safe, durable infrastructure. On the FE Environmental exam, link chemistry in the environment to the physical integrity of the systems you design.

Composite Materials and Linings

Fiber-reinforced plastic (FRP) tanks and piping resist chlorine and many acids. Lining systems (epoxy, vinyl ester, rubber) rehabilitate steel and concrete without full replacement. Specify compatibility with cleaning chemicals (hypochlorite, caustic) used in the facility.

Thermal Expansion and Joint Design

Buried PVC and HDPE expand with temperature swings. Bell-and-spigot joints, expansion loops, and proper backfill prevent buckling. Above-grade chemical piping needs anchor and guide spacing per manufacturer data.

Material Testing on FE Items

Exam scenarios may present tabular corrosion rates or chemical compatibility charts. Read the table axis labels — temperature and concentration change acceptable materials. When two materials are acceptable, total life-cycle cost and maintainability break ties, not chemistry alone.