We often think of the coating as the most significant factor in the protection of a surface. However, we should also consider the substrate (surface) and its effect on the stability, durability and effectiveness of the coating. The substrate, or surface over which the coating is applied, is the groundwork or foundation of the coating, so its characteristics have a direct influence on the life of the coating.

The construction of a building on sand, clay, or rock shows a similar relationship. The same house could be built on each of these bases, but the one built on clay would have a shorter life and therefore be less satisfactory than the one built on rock. Also, the foundation necessary for a house built on sand would differ from that needed to build on clay.

In the same way, coating systems vary according to the substrate, both the type of substrate material and how it is prepared before being coated. The performance of the paint system is affected by the surface type and its preparation prior to painting. If a coating has been developed to protect a particular type of material, it may not be suitable to use on a different type of material. Likewise, if a coating has been developed to be used on a blast cleaned steel surface, it is not likely to perform well on steel which has only been subjected to wire brushing.

We usually consider the preparation of a substrate before coating to consist of two phases:

1. Steel dressing, where the shape of the steel is under scrutiny, for example the presence of sharp edges, delamination in the steel, unsuitable welding seams, etc., and correcting these as necessary
2. Surface preparation, where the surface of the steel is scrutinized and improved so it conforms to the requirements of the coating to be applied in terms of cleanliness, roughness, etc.

Important factors to be considered for steel protection are:    

• Treatment of steel imperfections (Steel Dressing)
• The presence of rust and mill scale (Surface Treatment or Surface Preparation)
• The presence of surface contaminants, including salts, dust, oils and greases (Surface cleaning or degreasing)
• The surface profile

A clean and dry surface will provide the best adhesion for paint

Determining surface preparation methods

ISO 8504: Preparation of steel substrates before application of paints and related products. Surface preparation methods

Various methods can be used for surface preparation. In order to determine the most appropriate method, the contractor/designer must be aware of:

• Metal condition (e.g. new or previously coated steel)
• Expected surface preparation grades and profiles (if any)
• Available equipment and personnel
• Coating system to be applied
• Regulations / laws
• Cost

Steel substrates may consist of new metal, previously coated metal, or shop primed metal

• New steel: must be checked for initial type of steel, initial rust grade, any chemical treatment (chromating, phosphating), presence of chemicals or contaminants
• Previously coated steel: existing coating must be checked for genetic type, adhesion, DFT, condition, contaminants, etc.
• Shop Primed Steel: type of shop primer must be identified, its condition checked for mechanical and welding damages, contamination, and compatibility to subsequent coating system

Surface Preparation prior to Paint Application

• Assess the initial condition
  • For new steel, the inspector can refer to ISO 8501
  • Four grades of steel surface condition (A, B, C & D) are defined
  • To assess coated steel is more complicated, ISO 4628 is commonly used for assessment
• Steel dressing
• Surface cleaning (degreasing)
• Surface preparation/treatment (abrasive blasting, hand or power tooling, water jetting, etc.): remove rust and mill scale
• De-glossing, and abrading of the surface of the existing coating and feathering edges at damaged areas
• Removing abrading and/or dust residues

Steel defects affect coating performance

Steel defects can cause three possible coating problems:

• Adhesion: A sharp edge or weld spatter can result in the coating not covering the sharp edge allowing water penetration under the film which can result in loss of adhesion after corrosion commences
• Wetting properties: Contamination on the surface can hinder or restrict the coating from wetting the surface and developing adhesion
• Coating stress: An uneven surface (peaks and valleys) can result in changes to the surface tension as the coating cures / dries resulting in weak spots or cracking after cure

By Steel Dressing we mean removing steel imperfections (such as welding spatters, sharp edges, blow holes, etc.) through grinding or welding

Steel Imperfections treated by Grinding

• Lamination, shelling, slivers, hackles: Remove using grinder
• Weld ripple/profile: Sharp profile peaks to be smoothed using grinder
• Weld spatter: Remove by hammer, grinder according to specified grade
• Flame cut: Remove rough edges using grinder to smooth and radius the edge
• Sharp edge: Round to specified radius (e.g. 2mm) by grinder
• Rolled-in extraneous matter: Remove using grinder

 

 

Steel imperfection may require welding

• Undercut: Weld undercuts require re-welding and grinding
• Weld porosity: Treat by either welding/grinding or enlarge the pore for paint penetration
• Pits and craters: Either welding/grinding or enlarge for paint penetration

 

 

Grinding a construction

• Mark areas where grinding is required
• Remove weld spatter
• Grind the welds
• Grind the notches
• It is important to carry out grinding to improve the lifetime of the paint system
• Grinding of Sharp Edges
  • Paint will not be able to develop an adequate film thickness over a sharp edge
  • Rounding the edges followed by stripe coating gives an even film thickness and will improve the lifetime of the paint system

 

Disc grinder:

 

 

Pencil type grinder: 

 

 

 

Reminders

• Weld beads, spatter and other defects must be ground to form an acceptable substrate for the paint system
• Paint can’t fill blow holes
• Blow hole often becomes visible only after paint is applied
  • Will lead to initiation of corrosion by allowing moisture to penetrate
  • Remedial actions: Remove the paint, re-weld and grind
  • Re-Apply the paint

 

NOTE: Small failures like this are difficult to find, but the consequences can become big.

There are two types of Contamination:

• Visible contamination, which may include:
  • Condensation
  • Dust
  • Oil/grease
  • Rust and mill scale
  • Marine Fouling

• Non-visible contamination:
  • Soluble salts

Why should contamination be removed?

• Most contamination lacks adhesion to the substrate
• Some contaminates such as oil/grease have a high surface tension making paint adhesion difficult or impossible
• Soluble salts can absorb water from the environment (hygroscopic action) which can lead to:
  • Rapid surface rusting (steel) on the treated surface
  • Osmotic blistering of the coating if applied over soluble salts
• All contamination will result in poor adhesion of the paint film

Dust is a contaminant and may come from scaffolding

• Spent abrasive which has not been cleaned up after blast cleaning may blow or fall into the wet paint during application, leaving a rough surface which may not be covered by the paint film and can initiate corrosion
• If paint is applied over dust, a weak point is formed in the paint film, air can be trapped, adhesion is reduced and corrosion will develop rapidly

Equipment can contaminate the Surface

• Oil contamination from pneumatic equipment (e.g. air-driven grinder) can take place if oil is leaking out of equipment, the equipment has been stored with oil, or there is a missing oil trap (air from compressor contains oil)
• Must be removed prior to surface preparation

Marking Pens & Chalk can contaminate the Surface

• There are only a few types of Marker Pen recommended for inspection of steel and coatings
• Paint detachment may be caused by using the wrong type of marker pen
• All Chalk marks must be removed from bare steel and coatings before applying new paint

Welding smoke is a contaminant

• Smoke created by welding contains soot, burned material and salts from the flux in the welding rods
• Welding smoke is water soluble and should be removed by water washing
• If not removed, osmotic blistering of the paint may occur
• Solvents will not remove welding smoke sufficiently

The coating inspector may cause contamination!

• Dirty work boots or clothing may cause contamination inside a tank or on a freshly prepared deck or floor
• Human perspiration contains soluble salts
• Even a human fingerprint may in some situations be considered as contamination
• Osmotic blistering occurs (after submerging in water) when paint has been applied on a surface contaminated with Water Soluble Salts

Surface cleaning (degreasing)

Common methods:

• Fresh water cleaning (preferably pressurized water)
• Steam cleaning
• Solvent cleaning
• Other methods agreed upon

Cleaning procedure - water soluble degreaser

1. Apply the cleaner / degreaser from below and upwards
2. Work systematically on all surfaces
3. Let the cleaner react, normally 1-5 minutes
4. Wash off from below and upwards
5. Final rinse from above and down

Degreasing by wiping the surface with thinners and rags is not recommended. This method will spread a thin film of contamination such as oil over a wider area and the solvents increase the risk of fire or explosion.

Degreasing with alkaline detergent

• Should always be carried out before blast-cleaning
• After the detergents have been applied and allowed to work for the recommended time, they must be removed by ”Low pressure water cleaning (LPWC)” (around 250 - 300 bar) or higher pressures

Removal of marine growth, salts and loose paint

• It is important to start the washing as soon as the ship is in the dry-dock
• If the growth and contaminants are allowed to dry on the substrate they will be much harder to remove
• Usually pressure of minimum 250 bar at the nozzle
• Some of the antifouling paint on ships’ hull may be removed during the cleaning process (e.g. self-polishing antifouling)
• Certain types of marine growth (e.g. barnacles) will often require additional scraping or grinding to be removed

Marine fouling on ship side

Water Quality: Potential risks from unknown sources

The quality of the water is important for washing and contamination removal, otherwise additional contamination may take place during surface cleaning

• Check the source of the water, where is it coming from (drinking water, lake or river, brackish water, desalination plants, bore-holes, etc.)?
• Addition of inhibitors or other chemicals (chlorine)?
• Re-circulation, re-used water? Cleaning, filtering before re-use?
• Certificates or analysis?

Potential contents:

• Salts
• Contaminations
• Particles

Potential paint failures:

• Osmotic blistering
• “Dust” after drying, causing loss of adhesion