Surface Coating for Corrosion Prevention (With PDF)

What is Surface Coating?

Surface coating decorates and protects the surface on which it is applied. It can be defined as a homogeneous mixture of pigments, binders, solvents, and additives. The surface coating covers the surface completely and serves as an anti-corrosive agent. The success or failure of any coating is influenced by the following factor:

• Substrate Condition
• Surface condition and method of application
• Environmental condition at which it is applied and expected to withstand during service
• And last but not least the quality of paint used

Why Coat a Surface?

• Decorates the surface on which it is applied.
• Protects the surface from rust
• Protects from micro-organisms like fungi and algae maintaining their original body.

RUST NEVER SLEEPS!!!

Corrosion of Steel

• Environmental Corrosion: Unstable condition of steel
• Chemical Corrosion: Chemicals like acids, and alkalies attack metal
• Corrosion Prevention: Isolation of metals from the corrosive environment by using paint coatings

Corrosion Control

• Using Inhibitive Primers Zinc phosphates/Chromate forms a passive layer with adhered rust.
• Using Sacrificial Primer. Indirect catholic Protection by Zinc in Zinc-rich primers
• Barrier Coatings. High DFT Coating System Isolates Surface from corrosive Environment

Composition of Paint or Surface Coat

The basic constituents of paints are

• Pigments 5 to 25%
• Binders 60 to 65%
• Solvents 15 to 25%
• Additives 1 to 5 %

The relative proposition of these ingredients can be varied to produce films with any desired physical and application characteristics

Pigments for Surface Coating

A finely divided powder that can disperse in media of various types to produce paints. It is insoluble in the medium. Important properties are

• Colour
• Tinting Strength
• Opacity
• Fastness to light
• Resistance to heat
• The oil absorption of pigment
• Particle size: Hiding Power, Gloss or smoothness, Rate of settling of pigment

Binders for Surface Coating

Binders are the heart of the paint system. Binders bind or cement the pigment particle into a coherent film that adheres to the substrate. The mechanical and resistive properties of the film are controlled very largely by the binder.

The durability of the paint depends on the quality and quantity of binder used!!!!

• Convert the liquid coating on application to a solid dry film.
• Provide gloss to film
• Making the coating adhere to the substrate
• Given the elasticity of the film
• Resistance to water, chemical, and abrasion
• Disperse the pigments and extenders
• Hold the pigment in suspension.

The Choice of Binder for Paint depends on the end use of the paint

Type of Binders

• Drying oils: Vegetable oils on exposure to air, convert from liquid to solid thru a process of oxidation. Can be a sole film former but most often mixed with resin
• Resins: Most surface coatings contain a synthetic resin-based film former. Most decorative paints are based on oil-modified resins.

Few Important Binders/Resins

ALKYD Resins

• Largest groups of synthetic resins.
• They are oil-modified polyester
• Good exterior durability
• Low alkali and water resistance.

AMINO Resins: Melamine and urea-formaldehyde

Epoxy Resins:

• Has excellent adhesion, hardness, chemical, and corrosion resistance
• Can be used to do high-build paint
• Poor exterior durability

Poly-amide Resins: Used as curing agents for epoxy resins.

Polyurethane Resins:

• Good resistance to high temp, chemical and acid resistance, good resistance to various gases, alkali resistance.
• Low resistance to solvents like ketones, esters

Chlorinated Rubber:

• It is one pack of thermoplastic.
• Have good Chemical resistance, and good acid and alkali resistance.
• Can be applied as high-build paint.
• Disadvantage: Poor resistance to high temp and solvents like ketones, aromatic HC

• Vinyl Resins:
• Cellulose Resins: Widely used in auto finishing
• Acrylic Resins: Possess good light fastness, good adhesion, and excellent durability.

Solvents for Surface Coating

The primary function of the solvent is to dissolve film formers, thereby consistency suitable for the application. Choice solvent influences viscosity, drying and flow, and leveling.

Solvents are lost in the atmosphere, so it is an economic loss.

Solvents, in isolation or combination, are used in making thinner for the paints. 

Examples of solvents:

Hydrocarbon Solvent: Aliphatic, aromatic, solvent Naptha, alcohols, ketones, esters, etc.

Additives for Surface Coating

Used in a small amount to give a coating one or more desirable properties. The only difference between additives and other raw materials is that the amount of additives is very small. Properties that can be controlled thru additives are:

• Viscosity
• Setting 
• Drying
• Gloss 
• Opacity 
• Bacterial action
• Thickness
• Deodorants, etc.

Classification of Paints and Surface Coatings

Paints can be classified based on:

• The Physical state: Liquid Paint and Stiff Paint
• The Thinner Used: Water thinnable and solvent thinnable
• The End used: Decorative and protective.
• Modes of film formation: Thermosetting and Thermoplastics
• The order of application: Undercoat and topcoat
• The extent of gloss: Glossy, semi-glossy, egg-shell. matt
• Modes of Film Formation: Film formation is either by thermosetting or thermoplastics.

Thermoplastic (Non -convertibles):

In these coatings when the paint is applied on a surface, the solvent evaporates living resin to its original form spread over the surface. So change is only physical and can be reversed to its original form by using thinner. E.g Chlorinated Rubber

Thermosetting (Convertible):

Chemical changes occur in the coating and dry film is different from its liquid state. Ex. Epoxy, alkyds, etc.

Surface Preparation

Surface preparation is the most important part of a coating system. The surface preparation of the coating system is what a foundation is for a building.

Surface Preparation of Steel

Some of the various methods of steel are

• Degreasing
• Hand tool cleaning
• Power tool cleaning
• Flame Cleaning
• Pickling
• Abrasive Blast Cleaning
• Wet Abrasive Blast Cleaning

International Standard of Blast Cleaning

Paint or Surface Coat Application

Proper application of the protective coating is an important criterion in giving the paint system. Four main application procedures:

• Brush Application
• Roller Application
• Conventional Spray
• Airless Spray

Theoretical Coverage (Sq.M/Ltr) =(%Volume Solids X100)/DFT in microns