condition assessment and corrosion control for · pdf filecondition assessment and corrosion...

CONDITION ASSESSMENT

AND CORROSION CONTROL

FOR

CAN-TYPE PUMP STATIONS

Douglas Sherman, P.E., Principal Consultant

Randy Nixon, Principal Consultant

2017 WEAT Biosolids and Odor and Corrosion Control Conference

OVERVIEW

• Can-type Pump Stations – Construction

and Installation

• Corrosion Mechanisms and Causes

• Identifying and Monitoring these Problems

• Corrosion Repair & Mitigation Methods

• Summary

CONSTRUCTION AND

INSTALLATION

CONSTRUCTION AND

INSTALLATION

• Prefabricated

• Epoxy coating – internal & external

• Commonly designed with CP (mag

anodes)

• Installed quickly

CORROSION MECHANISMS

AND CAUSES


AND CAUSES

• Internal corrosion

– O2/moisture-driven

– Controlled with coatings

• External corrosion

– At breaches in coating

– Installed CP not beneficial


AND CAUSES

• Installed by developers who don’t

know, care

• Coatings not applied with great

quality (esp. at welds)

• Backfill disrupts coating

• Dry well not electrically isolated

• Anodes not connected

UNDERSTANDING THE CORROSION

OF UNDERGROUND, FERROUS METAL

STRUCTURES & PIPING

EXTERNAL (SOIL-SIDE)


• Electrolytic – Oxygen-driven

• Stray Current

• Graphitic – Cast iron, ductile iron

• Galvanic – Dissimilar metals, coupled

• Microbiologically Influenced Corrosion

(MIC)



Electrolytic – Oxygen-driven



Electrolytic – Oxygen-driven

• Microstructural composition differences in metal

surfaces produce anodic, cathodic sites

• Can also result from differential soil conditions

• Resulting potential differences

cause corrosion current to

flow from anodes to cathodes

• Corrosion rate influenced by

moisture content, oxygen,

resistivity, chemical

contamination



Stray Current

• PS near cathodically protected gas mains or 3rd-rail

DC train systems

• Driven by stray DC current leaving the steel PS to

return to circuit

• Typically involves a few

isolated or one deep pit at

the location of stray current

departure

• May be confused with

electrolytic corrosion



Anode-to-Cathode Area Relationship:

The smaller the anodic area relative to the

cathodic area, the higher the corrosion rate.



Concentration of Corrosion

at Coating Breaks & Pinholes

Rate of penetration related to

area from which metal is taken

IDENTIFYING AND

MONITORING CORROSION

IDENTIFYING AND


Visually inspect walls, floor, welds

• Corrosion

• In-leakage of ground water

• If false floor, gain access and examine floor

plates

IDENTIFYING AND


Ultrasonic Thickness Testing of Shell,

Floor Plates

• Need original thicknesses

• If false floor, gain access and UT floor plates

IDENTIFYING AND


Structure-to-soil Potential

Measurements

• To determine if structure is corroding (-300 to

-500 mV vs. Cu-CuSO4)

• To check original cathodic protection

IDENTIFYING AND


Is Corrosion Likely in the Future?

• Obtain soil samples and test/assess for

corrosivity

Soil Description Smooth, moist, brown, dense clumps with organic matter.

Resistivity as-received (Ω-cm) 1,600

Resistivity soil/water paste (Ω-cm) 1,300

Water Content (%) 15

pH 7.0

Alkalinity (mva1/kg) 1.0

Acidity (mva1/kg) N/A

Redox (mV, platinum vs. Ag/AgCl) +190

Chloride (mg/kg) 120

Sulfate (mg/kg) 387

Sulfide (yes/trace/no) no

Particle Size Analysis

% Sand

% Clay

% Silt

45.3

30.3

24.4

Soil Classification Clay Loam

SOILS CORROSIVITY ANALYSIS

Soil Test Results

SOILS CORROSIVITY ANALYSIS

IDENTIFYING AND




corrosivity

• Know ground water fluctuation range and

elevations

• Check electrical isolation from connected

piping

IDENTIFYING AND




corrosivity

• Know ground water fluctuation range and

elevations

• Check electrical isolation from connected

piping

• Check/measure for stray current effects

CORROSION REPAIR &

MITIGATION METHODS

CORROSION REPAIR &

MITIGATION METHODS

Localized, Deep Corrosion

• Install lap-welded patch on

interior

• Repair coating

CORROSION REPAIR &

MITIGATION METHODS

Existing Corrosion or Potential for

Future Corrosion

• Rely on original coating and CP system (rare

for original CP system to be effective)

• If in good-to-fair condition, use monitoring

approach

CORROSION REPAIR &

MITIGATION METHODS

Existing Corrosion or Potential for

Future Corrosion

• If in poor or questionable condition, install

new CP system (determine if should be

sacrificial anode or impressed current)

• If structurally compromised, excavate and

replace or make localized weld repairs; and

install CP

CORROSION REPAIR &

MITIGATION METHODS

The Maxim of Corrosion:

Anodes corrode.

Cathodes don’t.

Make a structure sufficiently cathodic,

and corrosion will stop.

CORROSION REPAIR &

MITIGATION METHODS

Steel Structure Protection with

Coating but no CP

CORROSION REPAIR &

MITIGATION METHODS

Steel Structure Protection with

Coating and CP

Sacrificial Anode

Cathodic Protection

VS

Impressed Current

Cathodic Protection

CORROSION REPAIR &

MITIGATION METHODS

CP – Sacrificial Anode System

CORROSION REPAIR &

MITIGATION METHODS

CORROSION REPAIR &

MITIGATION METHODS

CP – Impressed Current System

SIMPLIFIED GRAPHIC:

I.C.C.P. SYSTEM for

CAN PUMP STATION

TYPICAL ICCP SYSTEM LAYOUT

SUMMARY

• Reasonable condition assessment and

repair/mitigation approach without

excavation or interruption of operation.

– $15 to $25K to repair vs. couple $100K to

replace.

• For corroding (or potentially corroding)

can-type PS, if CP is properly designed

and maintained, it will extend life of PS,

and mitigate any future corrosion.

Thank you!

Any Questions?

condition assessment and corrosion control for · pdf filecondition assessment and corrosion...

Documents