clarifying material selection: understanding the …

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


Mechanisms


Corrosion Rates Affected by:

pH – normally 6 to 8; CO2 (covered tanks, bacterial metabolism)

Conductivity – sulfate, sulfide, chloride (coastal, ferric chloride)

Location


Mechanisms

Higher

Higher

Lower

Lower


Mechanisms


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)


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


Coated Steel

Organic resin-based

Epoxy

Polyurethane

Replaced coal-tar epoxy

Water, chemical resistance

Film build (12 to 30 mils/coat)


Coated Steel

Full epoxy below waterline

Epoxy/polyurethane above waterline

UV light resistance

Color/gloss retention


Coated Steel

Barrier protection

Minimize pinholes, discontinuities

Difficult in clarifiers: steel shapes

Angles, channel, flanged – edges, corners, etc.


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


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


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


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


HDG Steel

Other factors

O2

CO2

TDS

Chloride

Temperature

Agitation


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


Stainless Steel

Protective oxide film

Resists general corrosion

Susceptible to localized corrosion (pitting, crevice, MIC)

Threshold chloride concentration

PREN


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


Stainless Steel

Fabrication issues

Welds


Stainless Steel

Fabrication issues

Surface finish


Stainless Steel

Fabrication issues

Crevices


Stainless Steel

Process conditions

Wet/dry cycles


Stainless Steel

Process conditions

Microbial action

Stagnant conditions (< 5 ft/s)

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


Other Materials Considerations

Galvanic effects

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?

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