CLARIFYING MATERIAL SELECTION: UNDERSTANDING THE CORROSION MECHANISMS FOR CLARIFIER MECHANISMS

Transcription

1 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

2 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

3 Introduction

4 Understanding the Corrosion Mechanisms General Clarifier Environment (Immersion) Corrosion of steel is mainly oxygen-driven Generally < 10 mpy in aerated 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

5 Understanding the Corrosion Mechanisms General Clarifier Environment (Immersion) Corrosion Rates Affected by: ph normally 6 to 8; CO 2 (covered tanks, bacterial metabolism) Conductivity sulfate, sulfide, chloride (coastal, ferric chloride) Location

6 Understanding the Corrosion Mechanisms Higher Higher Lower Lower

7 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)

8 Understanding the Corrosion Mechanisms Above the Waterline Weathering exposure, high humidity Primary Clarifiers Biogenic sulfide corrosion biogenesis of H 2 S by SOB (covered tanks) Secondary Clarifiers H 2 S and other sulfur species mostly gone Oxygen-driven

9 Coated Steel Organic resin-based Epoxy Polyurethane Replaced coal-tar epoxy Water, chemical resistance Film build (12 to 30 mils/coat)

10 Coated Steel Full epoxy below waterline Epoxy/polyurethane above waterline UV light resistance Color/gloss retention

11 Coated Steel Barrier protection Minimize pinholes, discontinuities Difficult in clarifiers: steel shapes Angles, channel, flanged edges, corners, etc.

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13 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

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15 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

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18 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

19 HDG Steel Corrosion of zinc in water depends on ability to form CaCO 3 scale hydrogen ion concentration (ph; 6 12) total calcium content total alkalinity CaCO 3 stability Langelier Index Ryznar Index Practical Saturation Index

20 HDG Steel Other factors O 2 CO 2 TDS Chloride Temperature Agitation

21 HDG Steel 4 6 mils, typ. Pure Zn layer only 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

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25 Stainless Steel Protective oxide film Resists general corrosion Susceptible to localized corrosion (pitting, crevice, MIC) Threshold chloride concentration PREN

26 Stainless Steel Stainless Steel Grade UNS Number Cr % (Typ.) Mo % (Typ.) PREN 1 Approx. Cl - Concentration Below Which Pitting does not Occur (ppm) 2 Relative Cost (304 = 1.0) 304L S L S LMN S S AL6XN N Seawater PREN = Pitting Resistance Equivalent Number; %Cr %Mo + 16 %N, based on minimum composition 2 At 95 F, neutral ph

27 Stainless Steel Fabrication issues Welds

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30 Stainless Steel Fabrication issues Surface finish

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32 Stainless Steel Fabrication issues Crevices

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34 Stainless Steel Process conditions Wet/dry cycles

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37 Stainless Steel Process conditions Microbial action Stagnant conditions (< 5 ft/s)

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40 Stainless Steel Stainless Steel Grade UNS Number Cr % (Typ.) Mo % (Typ.) PREN 1 Approx. Cl - Concentration Below Which Pitting does not Occur (ppm) 2 Relative Cost (304 = 1.0) 304L S L S LMN S S AL6XN N Seawater PREN = Pitting Resistance Equivalent Number; %Cr %Mo + 16 %N, based on minimum composition 2 At 95 F, neutral ph

41 Other Materials Considerations Galvanic effects

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43 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

44 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

45 Summary Any Material Requires: Tight and enforceable material/fabrication spec Good QA/QC during fabrication and erection

46 Questions?