CLARIFYING MATERIAL SELECTION:
UNDERSTANDING THE CORROSION
MECHANISMS
FOR CLARIFIER MECHANISMS
DOUG SHERMAN, P.E. PRINCIPAL CONSULTANT, CORROSION PROBE, INC.
RANDY NIXON, PRINCIPAL CONSULTANT, CORROSION PROBE, INC.
WEAT Biosolids and Odor and Corrosion Conference
San Marcos, TX
August 5, 2015
Presentation Outline
Introduction
Understanding the Corrosion Mechanisms
General Clarifier Environment
Materials and Their Specific Issues
Coated Steel
Hot-Dip Galvanized Steel
Stainless Steel
Other Materials Considerations
Galvanic Effects
Summary
Introduction
Understanding the Corrosion
Mechanisms
General Clarifier Environment (Immersion)
Corrosion of steel is mainly oxygen-driven
Generally < 10 mpy in aerated wastewater @ 100°F, pH 6 – 8
Stainless steel would not corrode
Where not aerated, acidic environs form
Under deposits, stagnant areas
Sulfide, other ions
Mircrobial reactions
Understanding the Corrosion
Mechanisms
General Clarifier Environment (Immersion)
Corrosion Rates Affected by:
pH – normally 6 to 8; CO2 (covered tanks, bacterial metabolism)
Conductivity – sulfate, sulfide, chloride (coastal, ferric chloride)
Location
Understanding the Corrosion
Mechanisms
Higher
Higher
Lower
Lower
Understanding the Corrosion
Mechanisms
General Clarifier Environment (Immersion)
Corrosion Rates Affected by:
pH – normally 6 to 8; CO2 (covered tanks, bacterial metabolism)
Conductivity – sulfate, sulfide, chloride (coastal, ferric chloride)
Location
Differential cells (aeration, concentration) – potential differences
(anode/cathode)
Understanding the Corrosion
Mechanisms
Above the Waterline
Weathering exposure, high humidity
Primary Clarifiers
Biogenic sulfide corrosion – biogenesis of H2S by SOB (covered
tanks)
Secondary Clarifiers
H2S and other sulfur species mostly gone
Oxygen-driven
Materials and Their Specific Issues
Coated Steel
Organic resin-based
Epoxy
Polyurethane
Replaced coal-tar epoxy
Water, chemical resistance
Film build (12 to 30 mils/coat)
Materials and Their Specific Issues
Coated Steel
Full epoxy below waterline
Epoxy/polyurethane above waterline
UV light resistance
Color/gloss retention
Materials and Their Specific Issues
Coated Steel
Barrier protection
Minimize pinholes, discontinuities
Difficult in clarifiers: steel shapes
Angles, channel, flanged – edges, corners, etc.
Materials and Their Specific Issues
Materials and Their Specific Issues
Coated Steel
Barrier protection
Minimize pinholes, discontinuities
Difficult in clarifiers: steel shapes
Angles, channel, flanged – edges, corners, etc.
Proper selection, application
Edge retention
Film build/coat
“Stripe coating”
Attention to detail in shop and field
Materials and Their Specific Issues
Materials and Their Specific Issues
Coated Steel
Performance
15 to 18 years before major coating repair or recoating
Ongoing inspection, frequent repairs extend life to 30 years
Lower life-cycle cost
Materials and Their Specific Issues
Materials and Their Specific Issues
Materials and Their Specific Issues
Hot-Dip Galvanized (HDG) Steel
Galvanic protection
Anodic to steel
Barrier coating
Atmospheric: zinc → zinc oxide → zinc hydroxide → zinc carbonate
Affected by moisture contact, rate of drying, exposure to
corrodents
“White rust”
Immersion: protective layer of calcium carbonate
Materials and Their Specific Issues
HDG Steel
Corrosion of zinc in water depends on ability to form CaCO3 scale
hydrogen ion concentration (pH; 6 – 12)
total calcium content
total alkalinity
CaCO3 stability
Langelier Index
Ryznar Index
Practical Saturation Index
Materials and Their Specific Issues
HDG Steel
Other factors
O2
CO2
TDS
Chloride
Temperature
Agitation
Materials and Their Specific Issues
HDG Steel
4 – 6 mils, typ.
Pure Zn layer only 1.5 – 2
mils
When Zn-Fe layers exposed,
→ steel corrosion factors
Eta 100% Zn
Zeta 94% Zn 6% Fe
Delta 90% Zn 10% Fe
Gamma
75% Zn 25% Fe
Steel
Materials and Their Specific Issues
Materials and Their Specific Issues
Materials and Their Specific Issues
Materials and Their Specific Issues
Stainless Steel
Protective oxide film
Resists general corrosion
Susceptible to localized corrosion (pitting, crevice, MIC)
Threshold chloride concentration
PREN
Materials and Their Specific Issues
Stainless Steel
Stainless
Steel Grade
UNS Number
Cr % (Typ.)
Mo % (Typ.) PREN1
Approx. Cl-
Concentration Below
Which Pitting does not Occur (ppm) 2
Relative
Cost (304 = 1.0)
304L S30403 18 0 18 300 1.00
316L S31603 17 2.1 23 1000 1.27
317LMN S31726 18.5 4.1 32 5000 2.28
2205 S32205 22.5 3.3 34 5000 1.24
AL6XN N08367 20.5 6.1 43 Seawater 3.66
1 PREN = Pitting Resistance Equivalent Number; %Cr + 3.3·%Mo + 16·%N, based on minimum composition 2 At 95°F, neutral pH
Materials and Their Specific Issues
Stainless Steel
Fabrication issues
Welds
Materials and Their Specific Issues
Materials and Their Specific Issues
Materials and Their Specific Issues
Stainless Steel
Fabrication issues
Surface finish
Materials and Their Specific Issues
Materials and Their Specific Issues
Stainless Steel
Fabrication issues
Crevices
Materials and Their Specific Issues
Materials and Their Specific Issues
Stainless Steel
Process conditions
Wet/dry cycles
Materials and Their Specific Issues
Materials and Their Specific Issues
Materials and Their Specific Issues
Stainless Steel
Process conditions
Microbial action
Stagnant conditions (< 5 ft/s)
Materials and Their Specific Issues
Materials and Their Specific Issues
Materials and Their Specific Issues Stainless Steel
Stainless
Steel Grade
UNS Number
Cr % (Typ.)
Mo % (Typ.) PREN1
Approx. Cl-
Concentration Below
Which Pitting does not Occur (ppm) 2
Relative
Cost (304 = 1.0)
304L S30403 18 0 18 300 1.00
316L S31603 17 2.1 23 1000 1.27
317LMN S31726 18.5 4.1 32 5000 2.28
2205 S32205 22.5 3.3 34 5000 1.24
AL6XN N08367 20.5 6.1 43 Seawater 3.66
1 PREN = Pitting Resistance Equivalent Number; %Cr + 3.3·%Mo + 16·%N, based on minimum composition 2 At 95°F, neutral pH
Materials and Their Specific Issues
Other Materials Considerations
Galvanic effects
Materials and Their Specific Issues
Summary
For New or Rehab Design, Must Understand:
Operating environment(s)
Candidate materials and their properties
Damage mechanisms caused by interactions between them
Fabrication and erection practices and effects on corrosion
resistance
Coated Steel
Viable
Inspection and maintenance
Summary
HDG Steel
High susceptibility for failure in many WW environs
Metallic → more issues than organic coatings
Inspection and maintenance
Cannot be replaced
Stainless Steel
Step above in most cases
Higher initial material costs offset by lower inspection and repair costs
Can have issues ($$) if not chosen properly
Summary
Any Material Requires:
Tight and enforceable material/fabrication spec
Good QA/QC during fabrication and erection
Questions?