Cp for Water Facility

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CATHODIC PROTECTION FOR PORT FACILITIES

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Contents
• • • • • • What causes corrosion What is CP and how does it work? Galvanic vs. impressed current Electrolytes (or where can I use CP) Marine applications Condition assessments

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What is Corrosion?

Corrosion is defined as the degradation of a material due to a reaction with it’s environment. Metals require energy to process them from their native state to a usable material. Nature wants to reverse this.

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Corrosion Process

• Normal concrete has a very high pH (alkalinity) and this provides protection against corrosion of the embedded steel (passivation) • Chlorides and other salts, over time can penetrate the concrete matrix reducing the pH causing the reinforcing steel to

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Good Concrete

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Corrosive Environment
De-icing salts, coastal chloride deposition, heat, humidity, CO 2, F/T

_____________________________CORRTECH Typical Areas of Concern • Coastal structures
• Highways, Bridges • Sea Walls and other Marine Structures • Industrial Facilities

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The Corrosion Process

• Once the chlorides penetrate to the reinforcing steel, the concrete’s high pH no longer passivates the steel. • A typical galvanic corrosion cell forms and corrosion can occur quite quickly.

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Electrolyte Fe++ H0 H0 Conventional Current Flow H0 H0

H0

2eAnode
Electron Current Flow Metallic Path

Cathode

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The Corrosion Damage Begins
Rust expands volume 5-10x’s Internal stress + 5,000 psi
Electron Migration (e-)

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Corrosion Cell Elements
• • • • Anode Cathode Metallic path Electrolyte

Eliminate any one of the four (4) elements, and you stop the corrosion reaction with no further corrosion

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Cathodic Protection
Anode
Electrical Current

Metallic Path

Electrolyte Cathode

Cathodic Protection is an electrochemical process in which a DC current is applied to a metal to mitigate or off-set corrosion currents.

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Accelerated Low Water Corrosion • Tiny sulfate reducing micro-organisms (SRBs) grow in colonies. • Most favorable conditions are at the mud line and low water line. • Bacteria reduces sulfate common in seawater to create H2S which reacts with the steel to create iron sulfide • Iron sulfide is much more cathodic than the steel structure creating a strong corrosion cell.

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SRBs colonize at Low Water line and at the mud line – identified by orange brown surface color

SRBs create H2S which reacts with steel to form Iron Sufide – a grey black sludge which is very cathodic

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Fe++ Electrolyte Conventional Current Flow

2eAnode
Electron Current Flow Metallic Path

Cathode (Iron Sulfide)

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Cathodic Protection
Anode
Electrical Current

Metallic Path

Electrolyte
H0 H0 H0 H0
H0

Cathode
Application of cathodic protection reverses the strong corrosion cell generated by SRBs and creates hydrogen layer at metal surface inhibiting growth of SRB colony

_____________________________CORRTECH Electrolytes • Water – Key factors are temperature and salinity – Tidal variations for brackish water

• Soil
– Soil resistivity key factor – Other chemical contaminants

• Concrete
– Ingress of chlorides

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Types of CP Systems

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Galvanic vs. Impressed
• Galvanic (or sacrificial) passive system relying on the natural potential...
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