Chapter 1: Reasons for Coating

Coating

A liquid or liquefiable or mastic composition (thick film) that, after application to a surface, is converted into a solid, protective, decorative, or functional adherent film

4 Reasons for Coating

1. Aesthetics

2. Performance Requirements

3. Safety Identification

4. Corrosion Prevention + Control

Aesthetics

A set of principles that focus on the appreciation of beauty

Coatings have different functions based on…?

1. Location

2. Severity of Exposure

3. Length of Service Life Desired

4. Long-Term Corrosion Protection

True or False - No one coating can contain all of the performance properties

True

Chemical Resistance

Resist breakdown from the chemicals, primarily a function of the resin used

Water Resistance

Prevents or reduce the transmission of liquid or moisture or reduce the transmission of water vapor

Ease of Application

The easier the application, the less opportunity for defects to be created and premature failure

Adhesion to Substrate

Based on physical and chemical interactions between the coating and the substrate

Cohesive Strength

Coatings need to withstand the stresses of curing processes and changes in temperature and moisture content

Flexibility and Elongation

The ability to expand and contract with the substrate

Impact Resistance

Resist impact loads

Biocides

Resist attack by fungus, insects, rot, and mildew on wood and cementitious surfaces

Decontamination

Provide easier decontamination or cleaning of substrates

Abrasion Resistance

Reduce wear on the substrate from abrasion

Temperature Resistance

Resist extreme hot or cold temperatures

Marine Fouling

Keep marine surfaces smooth and reduce drag and marine organisms off the hull of ships and other submerged structures

Weatherproofing

Protection against degradation from the environment

Luminescence

Allow signs to be seen in the dark or in areas that could lose lighting

Passive Fire Protection

Resist the spread of fire

Safety Identification

Refers to signs and markings that provide viewers with information, directions and guidelines

Corrosion Prevention and Control (CPC)

prevents the start of or reduce corrosion through the use of coatings, cathodic protection, alteration of environment, materials selection, design, and inhibitors.

Corrosion

The deterioration of a metal or alloy that results from an electrochemical reaction with its environment.

The Corrosion Process

1. What Type of Metal?

2. How does the Metal React?

3. How does the reaction change the metal?

4. How does the reaction impair the metal?

The Corrosion Process: Step 1

Requires metal to be present

The Corrosion Process: Step 2

The metal reacts physically and chemically with it’s environment

Iron Oxide (Rust)

When iron or steel corrodes

Rust

Refers to the oxidation of iron

Oxidation (Rusting)

A chemical reaction that needs oxygen and water; occurs at the anode where the metal loses electrons

True or False - Oxygen is always present while water is present in most environments

True

Forging Metal - Basic Law of Nature

The more energy you add to a substance to change it into something else, the faster it wants to return to its original low energy state

A.C.M.E.

A - Anode

C - Cathode

M - Metallic Pathway

E - Electrolyte

The Corrosion Process: Step 3

The transformation of the metal into other chemical compounds with the formation of electrochemical cell

What does metal reactions result in?

Metal breaking down and forming iron oxide (rust)

Anode

The area of the metal surface that corrodes; the metal oxidizes (gains O2), loses mass and dissolves into the electrolyte as positive charged cations

Cathode

The area of the metal surface that DOES NOT corrode; the metal reduces (loses O2), forms negative charged anions

Reduction

A chemical reaction that occurs at the cathode that involves the gain of electrons to the metal

Metallic Pathway

Allows the passage of electrons between the anode and cathode

Electrolyte

Any solution or medium capable of conducting electricity (Ionic current)

Electrochemical Cell (Corrosion Cell or Circuit)

A condition on a metal surface where electric flows between to metal surfaces

The Corrosion Process: Step 4

The degradation of the metal, lost of strength, durability, hardness, structural integrity, thickness, etc.

Factors that Affect the Rate of Corrosion

1. Type of Metal

2. Type of Corrosion

3. Service Environment

Stability

The potential for the metal to react with it’s environment

True or False - The less stable a metal is, the more likely it is to corrode

True

True or False - The more stable a metal is, the less likely it is to corrode

True

Galvanic Series (Electro-potential Series)

This series lists materials by their reactivity or corrosion potential

Passivation

A layer of oxides formed on the surface of a metal that provides corrosion protection

Cathodic Protection

A prevention method that adds a sacrificial metal or anode to a system that then corrodes instead of the core asset

General Corrosion

Relatively uniform metal loss, general thinning of area, easy to visually inspect, and very rarely contributes to catastrophic failure

Localized Corrosion

Occurs at discrete sites, confined to a particular area, difficult to inspect, targeted thinning of area, often contribute to catastrophic failure

How does general corrosion form?

Multiple adenoids and paths existing at the same time all over the surface, the anodic and cathodic sites move

3 Types of Localized Corrosion

1. Dissimilar

2. Crevice

3. Pitting

Dissimilar Corrosion

Occurs when two metal/alloys with different potential come into contact within a corrosive electrolyte; The more active metal usually corrodes fast faster

Crevice Corrosion

Occurs in metal-to-metal contact, on a metal surface that is shielded from full exposure to the environment, and because of the proximity of another material that forms a narrow gap between them

Pitting Corrosion

Occurs at distinct spots where deep pits are produced, they can be isolated, or a group of pits may grow together to form a single larger pits

Service Environment

Refers to the characteristics of the location where the metal functions

Immersion

Submerged in a liquid

Atmospheric

Exposed to the air

Concentration of Oxygen

Oxygen increases the rate of corrosion, corrosion can happen in a O2 deficient environment = slower

Concentration of Water

Water increases the rate of corrosion, corrosion can happen without visible water = rate decreases below 60% Humidity

Naturally Occurring Salts

Salts increase the rate of corrosion by increasing conductivity of the electrolyte

What is the most common chemical salt?

Sodium Chloride

Atmospheric Moisture

Humidity (or wetness) - the wetter, the more likely corrosion is to occur

Manufactured Salts

Sulfate and nitrates derived from industrial combustion products and fertilizers that reduce the electrical resistance of the electrolyte

Temperature

Temperature increases = Corrosion increases

Earth Materials

Some soils cause more severe corrosion than others because the soil varies and composition and electric conductivity

How do coatings protect?

Codings creates a barrier between the environment’s electrolyte and the metal substrate

3 Protective Mechanisms

1. Barrier

2. Sacrificial

3. Inhibitive

Barrier Coating

A chemical and physical barrier that prevents the migration of an electrolyte but cannot eliminate moisture permeation and contaminants

Sacrificial Coatings

Coatings that are more active or oxidize more than the metal surface they protect

Inhibitive Coatings

A barricade that uses special pigments to actively slow down the reaction at the anode, cathode, or both

4 Coating Components

1. Pigments

2. Binder / Resins

3. Solvents

4. Additives

Pigments

Solid particles used to give specific protective, mechanical, or decorative qualities to a coating and hides the substrate

Binders (Resins)

Liquids that adhere to the substrate and chemically change to form a hard, protective film by bonding pigments

Curing

A coating’s conversion from a liquid to a solid

True or False - The binder is the only component of a coding that must be present, the other components are optional

True

Organic Coatings

Generally made from living or once-living organisms and contain carbon and hydrogen

Inorganic Coatings

Commonly based on silicone or silicates, non-living substances and do not contain carbon

Solvents

The liquid component of the coating that is responsible for dissolving the solid binder or resins

Solvency Power

The ability to dissolve the resin

Volatility

The speed at which the solvent leaves the coating film during and after application, this governs evaporation rate

True or False - Solvents emit, dangerous gases like VOCs or remit HAPs

True

Additives

Special purpose ingredients used to enhance the performance, certain properties or characteristics of the coating

How do electrons that are produced in oxidation reaction flow?

From the anode to the cathode through the metallic pathway

Vehicle

Binder + Solvent = Liquid portion of the coating

What are 3 coating properties that might be a priority to an underwater support system of this offshore rig?

1. Water Resistance

2. Marine Fouling

3. Weatherproofing

Galvanic Corrosion

The more active metal connected to the less active metal will corrode faster/preferentially

What’s an example of a Barrier coating?

Epoxy

What’s an example of an Inhibitive Coating?

Alkyd Primer

Name the 5 most important factors that affect the rate of corrosion

1. Humidity

2. Oxygen

3. Manufactured Salts

4. Temperature

5. Pollutants

What are two broad classifications of coatings?

1. Organic Convertible Thermosetting

2. Inorganic Non-Convertible Thermoplastic

What are two primary components of a liquid applied coating?

1. Pigments

2. Vehicle (Binder/Resin + Solvent)