Corrosion Protection of Steel by High Performance Coatings
To understand Corrosion Protection of Steel, it is essential to understand Corrosion Electrochemistry involved and elements causing the corrosion. In a lay language, corrosion of steel in its simplest manner, takes place due to presence of moisture and oxygen. It is exhibited in the diagram below. There are anodic & cathodic sites on steel surface. In presence of surface water or other conducting solution, electrons flow from anode to cathode giving rise to electron deficient anodic sites and there by generation of ferrous ions (Fe+2). At cathode Oxygen, moisture & electrons form hydroxyl ions. Finally, Hydroxyl ions and ferrous ions react with each other to form ferrous hydroxide. Further reaction of ferrous hydroxide with oxygen & water takes place to form ferric oxide hydrate (Rust).
Corrosion / Rust formation rate increases rapidly when electrons from anode to cathode move without obstruction. i.e. Passage of electrons is rapid in presence of conductive water which has electrolyte in it. This condition is possible when there is salinity in surface water by dissolved salts or by dissolved chemical pollutants from atmosphere. Therefore, corrosion rate is very high in marine (source of salts & condensed water), humid (water source) and Industrial (Pollutants like oxides of sulphur & nitrogen, chlorides, sulphates etc.) environments.
Other reasons for rapid corrosion of steel include direct reaction of metal surface with corrosive chemicals, Heat and Stress cracking etc. Stress cracking is combined effect of tensile stress and corrosive environment.
Corrosion Protection of steel can be done by Use of Coatings, Cathodic protection (e.g. Galvanizing of steel sheets), Impressed Current Cathodic Protection (e.g. Ship hull or gas & oil lines), Engineering Design (e.g. of Chemical tanks), Inhibitors (e.g. Amines used in petroleum Extraction & Refining as an corrosion inhibitor) and Selection of material etc.
Since ‘Corrosion Protection of steel by High Performance Coating’ is the topic of this article, all other corrosion protection measures are out of scope of discussion here. For a systematic approach, it becomes imperative to know ‘Corrosion Protection’ standards related to coatings available internationally. There are various ISO standards available like ISO 9223, ISO 9224 and ISO 12944 (8 parts) etc.
On the basis of above standards, following Durability of coatings are in practice,
1. Low Durability – 2 to 5 Years
2. Medium Durability – 5 to 15 Years
3. High Durability – More than 15 Year
Paint Schemes for Different Environments for Steel Structures under ISO 12944 – Part 5 are available.
For example, Typical Medium Durability Paint Schemes for Very High Marine & Immersion (Sea Water) Environments are as given in Table-5:
# High Solids
With advent of technology, apart from environmental considerations the demand for Paints & Coatings with superior properties and higher service life resulted in the usage of innovative modified polymeric structures for various coating technologies. Inorganic zinc silicate primers are available which outperforms epoxy primers. Acrylic aliphatic Polyurethane coating as a top coat gives better performance than Polyester-Aliphatic PU. There are many modified formulations in Epoxy, Acrylic and Polyurethanes are available which yield better service life, ease of surface preparation, and reduction in application cost and so on.
Guidelines given above under ISO 9223; 9224 & 12944 are useful in deciding a minimum Paint scheme required for given location for steel. However user specific other factors like Extent of surface preparation possible, expected wear & tear of coatings (Mechanical Damages expected), Condensation of moisture on surface, Cyclic wetting, activities generating heat around coating surface etc make user to select a higher performance rated coating system against suggested by ISO standard or extra care is required by having specified system with higher DFT. Therefore, above coatings chart of Durability vis-à-vis Paint System requirement should be treated as just guidelines.
Because of new developments, conventional polymers like Unmodified Alkyds, Chlorinated Rubber, Coal tar epoxies etc. are progressively being replaced by Modified Alkyds & Epoxies, Polyurethanes, Silicone & modified silicones etc.
To understand current market scenario with respect to Customer requirements in terms of Service life expected, Extent of surface preparation, Cost, Maintenance periodicity, Aesthetics and Environmental issues etc., it is essential to know about various High-Performance products and their typical properties.
Development of High-Performance Coatings - Driving Factors
1.Extent of Surface Preparation
- Dry Blasting always not possible
- Hydro Blasting always not possible
- Power tool / Hand tool cleaning
- St2 / St3 cleaning for Surface tolerant coatings
2. Flexibility in Application – Airless/Conventional spray/Brush/Roller
3. Recoating of subsequent coats in shortest time lapse
4. Compatibility with existing/aged coatings
5. Meeting Environmental regulation – e.g. VOC (Volatile Organic Component)
6. Life of Coating
Types of Paints & Coatings based on Resin/Polymer Chemistries:
Air Drying - Normal Setting / Drying (1K):
• Alkyd Based – Zinc chromate primer (IS/1874) & Full Glossy Finish (IS- 2932)
• Etch Primer or Adhesion Promoter – For Aluminium substrate
• Fire Retardant Enamels – Based on Typical pigments / additives – It has low flame spreading.
• Intumescent Coatings – Fire Rating up to 1 Hour – Protects steel from reaching 550º C for given fire rating during fire. At this temperature steel starts losing strength. Thick adherent char is formed when coating exposed to fire & subsequently protects steel.
• Heat Resistant Aluminium Paint (Up to 600º C) – These are Silicone based products and are used for chimneys, boilers etc. Paint dissipates heat fast and protects substrate from corrosion also.
• Heat Resistant Paint without Aluminium (Up to 250º C) – Modified Silicone based Topcoats having various shades. Primer has to be Inorganic Zinc Silicate.
• Fire Retardant Paint – Protective coatings are modified with certain fireretardant components to make them fire retardant. e.g. Interior coatings of cabins on ship.
Air Drying - Quick Setting / Drying (1K):
• Etch Primer or Adhesion Promoter – For Aluminium substrate
• PVC/ PVA Copolymer Paint – Primer & Finish
• Chlorinated Rubber – Primer & Finish
• Modified Acrylic Based – Primer & Finish – Paints dry very fast which gives more application job per day. Single component.
• Pure Thermoplastic Acrylic – Primer & Finish – Fast drying with good overall properties. Single component and easy to handle
Air Drying & Chemical Reaction (2K)
1. Epoxy Enamel (IS-14209)
2. Epoxy High Solids (Requires Airless Spraying) – Low permeability. Therefore, less susceptibility to corrosion.
3. Epoxy MIO / Glass Flake (Barrier coats) – Very low permeability in glass flake epoxy. Ideal in offshore application.
4. Epoxy Solvent Free (Requires Airless Spraying) – Better version of Epoxy High Solids. Higher DFT in one coat and thereby saving in application cost. Tank internal coatings.
5. Epoxy Surface / Moisture Tolerant (Polysulphide Modified or Based on Specialty Hardeners) – St2/ St3 Surface preparation is acceptable. Suitable for Water Ballast Tanks. Low ambient temperature curing.
6. Epoxy Novolac (EPN) – High Chemical / Heat Resistance. Chemical tanks and equipments.
7. Coal Tar Epoxy (Cheaper coating – Recommended for under water / Ground). These coatings are being replaced with better options.
1. Aliphatic PU (Polyol – Alkyd, Polyester etc & Hardener – Aliphatic Isocyanate) - IS 13213
2. Aliphatic Acrylic PU – IS 13213 – Good weatherability in polluted environment. Gloss retention & good cosmetic coating.
3. High Solids Acrylic PU – Higher DFT per coat.
4. Aromatic PU – (Polyol – Alkyd, Acrylic, Polyester or Epoxy & Hardener – Aromatic Isocyanate). Specially recommended for High Degree of Chemical Resistance (Poor UV Resistance) when Acrylic or Epoxy polyols are used. Due to poor UV resistance these coatings are used as Primers & Undercoats.
Common Primers – Epoxy Red oxide, Zinc Rich (IS-14589), Epoxy Zinc Phosphate, Surface Tolerant & Moisture Tolerant Epoxy, Polyurethane, Inorganic Zn Silicate (IS-14946). Etch Primer or Adhesion Promoter for Aluminium Substrate
1. Adhesion Promoter:
Used on bare surface before primer coat on difficult surface like Aluminium, Copper, Zinc and Glass. Also used on concrete.
2. Zinc Based Primers:
Disadvantage -These primers are not Acid/Alkali resistant unless Top coated
Advantage – Cathodic Protection
Best Primer - Inorganic Zinc Silicate (IS-14946). Up to 75% Zinc in dry film. It requires professional applicators and cannot be applied below 50% Relative Humidity.
3. Rust Converter (IS-13515): Rusted steel structures where normal cleaning is not possible. There should not be loose rust & Surface should be uniformly. Rust is converted to stable Iron Phosphates.
# Poor Solvent Resistance
(*) Not Applicable
@ Suitable as Primers only
Finalizing a Paint Scheme:
Before finalizing the Paint Scheme its each product should be confirmed for convenience of application in terms of
- Extent of Surface Preparation Required
- Mode of Application and any Product specific care needed. e.g. Inorganic Zinc Silicate Primer cannot be applied when Relative Humidity (RH) is less than 50%.
- Total downtime required for entire work
- Environmental & Safety issues (if any) – e.g. In a confined location solvent-free or low VOC paints pose less health hazards.
While selecting products under specific scheme, get maximum information from supplier through Product Data Sheets & MSDS. For most of the typical products, National & International technical specifications are available from IS/BIS/ASTM etc. Specification gives fair idea about Composition, Physical parameters, Application, & Performance expected by the product.
e.g. Commonly used Protective Paints are:
- Exterior Synthetic Enamels - IS 2932
- Air Drying Zinc Chromate Primer. IS 1874
- 2K Epoxy Zinc Phosphate Primer - IS 13238
- 2K Zinc Rich Epoxy Primer - IS 14589
- Inorganic Zinc Silicate Primer - IS 14946
- 2K Epoxy Enamel - IS 14209
- 2K Polyurethane Paint - IS 13213
- Exterior Finish for Ships - IS 6951
- Etch Primer – IS 5666
- Rust Converter - IS 13515
- Heat Resistant Paint Aluminium - IS 13183
Apart from above generic products there are many other specialty products as discussed under ‘Types of Paints’ are available with enhanced properties. Technical data sheets need to be checked for end properties and requirements.
Surface Preparation & Application Tips:
- Manual Surface Preparation is not enough for Zn Silicate Primer. Minimum Sa 2½ Profile required
- Sa 2½ Profile is good for Epoxy / PU System
- After blasting, within 4 hours primer should be applied to avoid flash rust
- Substrate should be free from Grease, moisture & other contaminants
- Painting not to be done above 85% Relative Humidity (Except Zinc Silicate Primer)
- Substrate temperature should be 3ºC above Dew Point to avoid condensation of moisture over surface
- Oil and Water filters of compressor should be clean
- Mixing of Base & Hardener parts of 2K paints should be done thoroughly
- Epoxy – After mixing Base with Hardener- Induction time is required
- Remember Pot Life in 2K paints
- Use Paint and Thinner of same Manufacturer
- Maintain right pressure at Tip of gun and Tip size
Conventional – 1.2 mm to 1.5 mm / 45psi – 75psi
Airless – 0.015 inch to 0.022 inch / 1740psi – 2610psi
Conventional Spray – 25 to 30 sec @ 30 deg C (Ford Cup B4)
Brushing – 40 to 60 sec @ 30 deg C (Ford Cup B4)
Air-less Spray – 12 to 15 Stokes
- Overcoating Interval (Min/Max): Follow Manufacturer’s Instruction – 16 to 18 Hours (Min) / Max- no
Epoxy - 16 to 18 Hours (Min)/ Max- 5 Days
PU - 16 to 18 Hours (min)/ Max-5Days
- If overcoating is required after suggested max period as a special case, roughening of coating with sand paper is advised
- Cleaning of Equipment – Use appropriate thinner (Immediately)
- Define your environment (C1, C2 etc.)
- Define Durability (Low, Medium, High)
- Refer recommendation of System from national/international standard
- Ask supplier to suggest the system appropriate to your requirements
- Ask supplier to provide Technical Data Sheets
- Check for your technical constraints (if any)
- Finalize the system using available inputs
- Costing of Particular Paint Scheme should be compared on the basis of - Cost of paints & thinners per square meter (considering losses depending on mode of application), Cost of blast cleaning, labour cost & Service life. Total cost worked out for a given scheme should be compared as a Net cost per Square meter per year of Service life expected.
- Many a times costly paints are found to have cost advantage because of Enhanced Service life, Ease of application, Manual surface preparation instead of elaborate blast cleaning (Sa 2.5), higher anti-corrosive properties and possibility of application of higher DFT in one go etc.
Note: This article is issued for the Magazine ‘Sailors & Warriors’. Not to be copied, circulated or printed in any form without prior permission of the author.