surface preparation

INTRODUCTION :To obtain the best results from a paint system the best possible surface

preparation consistent with the degree of sophistication of the system is required.

The quality of the paint and surface preparation (i.e. the removal of rust, scale,

salt, grease, dirt and loose paint) are the most important factors in determining

the lifetime of the paintwork, and therefore the length of time the surface will be

protected. First class preparation has the potential to extend the protective life of

any paint system.

What is surface preparation?

Coating performance is directly affected by surface preparation.

Coating integrity and service life will be reduced because of improperly prepared surfaces.

As high as 80% of all coating failures can be directly attributed to inadequate surface preparation that affects coating adhesion.

The majority of paintable surfaces are concrete, ferrous metal, galvanizing and aluminum.

They all require protection to keep them from corroding in aggressive environments.

Selection of the proper method for surface preparation depends on the substrate, the environment, the coating selected and the expected service life of the coating system.

The greater the

DFT the greater

the Surface

Profile

SURFACE CONTAMINANTS Mill Scale :The complete removal of mill scale and rust is critical in order to limit its corrosive

properties. Mill scale is generally found on new steel as a hard brittle coating – it consists of

distinct layers of various iron oxides and is formed during the hot rolling process in the

manufacture of steel. Painting over mill scale is not satisfactory, as mill scale is not firmly or

permanently bonded to the steel. Eventually the scale will lift off, causing paint coatings to

flake and corrosion to occur.

Rust: Rust is a form of hydrated ferric oxide formed by the action of water and oxygen on

iron and steel surfaces and occupies a much greater volume than the original steel. It is for

this reason that rust forming beneath a paint layer is able to rupture a coating. All areas of

rust should be completely removed – any rust remaining will act as a nucleus for further

rusting.

Oil, Grease Fat and Perspiration : Oil, grease, fat and perspiration deposits cannot be

removed by mechanical methods of cleaning. The simplest method of removal is to use a

solvent emulsifiable degreaser or biodegradable detergent. Apply the degreaser liberally

to the contaminated surface, allow to stand for a short time, then hose off with fresh

water.

Prepare surface for subsequent processing, such as a coating application or adhesive

bonding

Improve hygiene conditions for workers and customers

Remove contaminants that might chemically react with the surface

Enhance appearance and performance of the product

Factors in Selecting a Cleaning

Method

Contaminant to be removed

Degree of cleanliness required

Substrate material to be cleaned

Purpose of cleaning

Environmental and safety factors

Size and geometry of the part

Production and cost requirements

Various contaminants build up on part surfaces, either due

to previous processing or factory environment

Principal surface contaminants found in factory

Oil and grease, e.g., lubricants in metalworking

Solid particles such as metal chips, abrasive grits, shop dirt,

dust, etc.

Buffing and polishing compounds

Oxide films, rust, and scale

Contaminant to be Removed

What are Non-Ferrous Metals?

Non-ferrous metals refer to metals that are not derived from iron ore, such as

aluminium and its alloys, copper, brass and bronze. Whilst iron (mild steel) will readily

oxidize to red rust (iron oxide), non-ferrous metals usually exhibit only tarnishing

(surface corrosion). Aluminum finds ready use in window- and door-frames, handrails,

balustrades and architectural work generally. Copper is used in water pipes and

guttering.

.

Primary Substrates

• Steel– Carbon Steel – Easily attacked by the

environment in which it is placed and will begin to revert back to iron ore.

– Stainless Steel - Stainless steel does not stain, corrode, or rust as easily as ordinary steel.

Non-Ferrous Metal SurfacesAluminum

Remove all oil, grease, dirt, oxide and other foreign material

by cleaning per SSPC-SP1, Solvent Cleaning.

Galvanized Metal

Allow to weather a minimum of six months prior to coating.

Clean per SSPC-SP1 using detergent and water or a

degreasing cleaner, then prime as required. When weathering

is not possible or the surface has been treated with chromates

or silicates, first Solvent Clean per SSPC-SP1 and apply a test

area, priming as required. Allow the coating to cure at least

one week before testing per ASTM D3359. If adhesion is poor,

Brush Blast per SSPC-SP7/NACE 4 is necessary to remove these

treatments.

Ductile Iron

National Association of Pipe Fabricators, Inc. www.napf.com NAPF 500-03 Surface

Preparation Standard for Ductile Iron Pipe and Fittings in Exposed Locations Receiving

Special External Coatings and/or Special Internal Linings This standard summarizes the

surface preparation requirements for ductile iron. Included within this standard are the

following:

NAPF 500-03-01 Solvent Cleaning

NAPF 500-03-02 Hand Tool Cleaning

NAPF 500-03-03 Power Tool Cleaning NAPF 500-03-04

Abrasive Blast Cleaning for Ductile Iron Pipe

NAPF 500-03-05 Abrasive Blast Cleaning for Cast Ductile Iron

NOTE:Recommended Coating Systems for Non-Ferrous Metals :

A surface tolerant epoxy primer is the preferred primer for most non-ferrous

metals.

ConcreteConcrete - used more than any other man-made material in the world and is easily attacked when placed in chemical environments.

PROPER SURFACE PREPARATION

Proper surface preparation includes the following:

1. Inspection of the concrete substrate

2. Removal and replacement of non-durable concrete

3. Decontamination of the concrete surface

4. Creation of surface profile 5. Repair of surface irregularities

.

METHODS OF SURFACE PREPARATION

Depending upon conditions of the concrete one or more methods of surface

preparation may be required. It is common for decontamination to precede

mechanical preparation, and if necessary a second decontamination to follow

Protection Mechanisms

• Barrier Protection– The coating/lining isolates the electrolyte from

the anode, cathode, and metallic pathway.

• Rust Inhibitive– The slightly water-soluble pigments permeate

to steel/coating interface and passivate the substrate.

• Sacrificial– The coating/lining contains pigments that are

more active than the metal and sacrifice themselves to protect the substrate.

Chemical Cleaning Processes

Alkaline cleaning

Emulsion cleaning

Solvent cleaning

Acid cleaning

Ultrasonic cleaning

Alkaline CleaningUses an alkali to remove oils, grease, wax, and

various types of particles (metal chips, silica,

light scale) from a metallic surface

Most widely used industrial cleaning method

Alkaline solutions include sodium and

potassium hydroxide (NaOH, KOH), sodium

carbonate (Na2CO3), borax (Na2B4O7)

Cleaning methods: immersion or spraying

followed by water rinse to remove residue

Emulsion Cleaning

Uses organic solvents (oils) dispersed in an aqueous

solution

Suitable emulsifiers (soaps) results in a two-phase

cleaning fluid (oil-in-water), which functions by

dissolving or emulsifying the soils on the part

surface

Used on either metal or nonmetallic parts

Must be followed by alkaline cleaning to

eliminate all residues of the organic solvent prior

to plating

Solvent Cleaning

Organic soils such as oil and grease are removed

from a metallic surface by chemicals that

dissolve the soils

Common application techniques: hand-wiping,

immersion, spraying, and vapor degreasing

Vapor degreasing (a solvent cleaning

method) uses hot vapors of chlorinated or

fluorinated solvents

Acid Cleaning

Removes oils and light oxides from metal surfaces

using acid solutions combined with

water-miscible solvents, wetting and emulsifying

agents

Common application techniques: soaking,

spraying, or manual brushing or wiping carried

out at ambient or elevated temperatures

Cleaning acids include hydrochloric (HCl), nitric

(HNO3), phosphoric (H3PO4), and sulfuric (H2SO4)

Acid Pickling

More severe acid treatment to remove thicker

oxides, rusts, and scales

Distinction between acid cleaning and acid

pickling is a matter of degree

Generally results in some etching of the metallic

surface which serves to improve organic paint

adhesion

©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Ultrasonic Cleaning

Mechanical agitation of cleaning fluid by

high-frequency vibrations (between 20 and 45 kHz) to

cause cavitation (formation of low pressure vapor

bubbles that scrub the surface)

Combines chemical cleaning and mechanical

agitation of the cleaning fluid

Cleaning fluid is generally an aqueous solution

containing alkaline detergents

Highly effective for removing surface contaminants

Phosphating:

Phosphating improves the adhesion of a the coating to

the metal surface and can also improve the corrosion

resistance of the painted surface. Iron, steel, zinc and

hot galvanized surfaces are suitable for phosphating,

and in some cases, Aluminium surfaces.

During phosphating a thin, crystalline phosphate

layer forms on the metal surface, which improves the

adhesion. The most usual phosphating methods are

iron and zinc phosphating.

ChromatingChromating is used as a pretreatment method for light

alloys and galvanized surfaces. The treatment causes

a thin colorless or yellowish coat. There are also

chromating methods which form a substrate which is not suitable for painting.

Chemistry:-

Chromate coating on aluminium may contain the compound

AL2O3.Cr(OH)3.CrOH.CrO4.

On zinc as basis metal, ZnCrO4 has been claimed to appear

Mechanical Cleaning

Physical removal of soils, scales, or films from the

work surface by abrasives or similar mechanical

action

Often serves other functions also, such as

deburring, improving surface finish, and surface

hardening

Processes:

Blast finishing

Shot peening

Mass finishing processes

Blast Finishing

High velocity impact of particulate media to clean

and finish a surface

Media is propelled at the target surface by

pressurized air or centrifugal force

Most well-known method is sand blasting, which

uses grits of sand as blasting media

Other blasting media:

Hard abrasives such as Al2O3 and SiC

Soft media such as nylon beads

Shot Peening

High velocity stream of small cast steel pellets

(called shot) is directed at a metallic surface to

cold work and induce compressive stresses into

surface layers

Used primarily to improve fatigue strength of

metal parts

Purpose is therefore different from blast

finishing, although surface cleaning is

accomplished as a byproduct of the

operation

Surface Preparation Affects on System Selection

• SSPC-SP 1 “Solvent Cleaning”

• SSPC-SP 2 “Hand Tool Cleaning”

• SSPC-SP 3 “Power Tool Cleaning”

• SSPC-SP 11 “ Power Tool Cleaning to Bare Metal”

• SSPC-SP 7/NACE No. 4 “Brush-Off Blast Cleaning”

• SSPC-SP 6/NACE No. 3 “Commercial Blast Cleaning”

Uses of pretreatment:-

To increase the long-life of a substrate.

To increase the coating performance.

To increase the surface area for good adhesion

To remove all contamination from the metal surface

and prevent their re-deposition.

Conclusion:

Pretreatment is a part of surface coating technology therefore before

application of protective coating pretreatment is must in order to

have improved properties of the coating and the substrate. It is

expected that technocrats in the finishing industry try to make use of

it and upgrade it as per the requirement.