Members Area
     
T: +44 (0)114 551 8170
E: ssas@bssa.org.uk

Paint coating stainless steels

Introduction

Stainless steels are used because of their corrosion resistance in a wide variety of service environments, usually without additional coatings. In certain circumstances, however, stainless steel components or structures may require a coated, (paint), finish. Examples of this may include company colour schemes or logos, environmental blending and compatibility, and general aesthetic requirements.

Coating systems for stainless steels must be carefully selected to provide sufficient durability to meet the requirements of the service environment, and its associated operating conditions. The combination of surface preparation and formulation of the paint system are key factors in meeting these objectives. In certain environments, localised breakdown of the coating can lead to corrosion, which may be more severe than that experienced with un-coated surfaces, and may result in high, localised, rates of attack.

In general, stainless steels have flatter and smoother surfaces than carbon steels. This applies particularly to thin cold-rolled products, (sheet and coil), and may adversely affect adhesion between the metal surface and the coating system. Roughening of stainless steel surfaces prior to coating is essential, and can usually be achieved by abrasive blasting, light hand abrasion or chemical etching.

Surface preparation

Abrasive Blasting

Abrasive blasting can be accomplished by using clean, fine, hard non-metallic abrasive particles, (e.g. alumina or silicon carbide). The abrasive medium must be iron-free to avoid contamination, which can result in rust staining on the surface prior to coating, and the compressed air carrying the abrasive medium must be free of compressor oil.

Abrasive blasting may be readily confined to specific areas by masking. Support should be given to thin sections or cold-rolled material to avoid distortion or damage during blasting.
Stainless steel surfaces should be prepared to give a surface roughness, Ra, of approximately 50 micron, and a minimum cleanliness of grade 2 in accordance with BS 7079:Part 1A.

Light Hand Abrasion

To avoid distortion to light sections, light hand abrasion is an alternative method to blasting. Iron-free, 320 to 400 grit abrasive media are usually satisfactory. Chemical etching treatments can also be considered. Certain paint manufacturers have special primers designed for such applications, and it is recommended that details of the surface to be coated are specified when seeking advice on an appropriate coating system.

Final Cleaning

Prior to coating, all stainless steel surfaces must be dry and free from rust, other foreign materials, oil and grease. The presence of such contaminants may lead to failure of the coating system. Oil and grease may be removed by organic solvents, for example acetone and final drying done with a hot air blower.

Coating application and systems

The table below gives details of paint systems suitable for coating stainless steels and, in accordance with ISO 12944, (but see below for how recent changes in this standard has affected this table), and ISO 9223, classifies them in terms of the severity of the envisaged service environment.

Exterior Environment Coating System
Category Corrosion Risk Location .
C3 Medium Rural and urban areas with low sulphur dioxide, acid, alkali and salt pollution Two pack epoxy or polyurethane primer suitable for stainless steel at 30-50 micron dry, FOLLOWED BY High solids polyurethane finish at 100 micron dry
C4 High Urban and industrial areas with moderate sulphur dioxide and/or coastal areas with low salinity Two pack epoxy or polyurethane primer suitable for stainless steel at 30-50 micron dry, FOLLOWED BY High build epoxy MIO* at 100 mm dry, FOLLOWED BY Re-coatable polyurethane finish at 60 micron dry
C5 Very High Industrial areas with high humidity and aggressive atmospheres and coastal areas with high salinity Two pack epoxy or polyurethane primer suitable for stainless steel at 30-50 micron dry, FOLLOWED BY High build epoxy MIO* at 200 micron dry (one or two coats), FOLLOWED BY Re-coatable polyurethane finish at 60 micron dry

MIO = Micaeous Iron Oxide

Cleanliness is extremely important in the successful application of paint coatings to stainless steels surfaces. Paint coatings should be applied in clean, dust-free conditions to clean, dry stainless steel surfaces.

If the correct precautions are taken, the complete coating systems suggested in the table can be applied ‘on-site’.

It may be preferable for some or all of the components of coating systems to be applied under paint shop conditions, where site conditions cannot be carefully controlled. Provided the initial coating stages are done under these conditions, then the final coat can be applied ‘on-site’ or after final assembly.

The first primer coat can be followed by subsequent coats in strongly contrasting colours to ensure complete coverage of the component or structure by the full coating system. The paint manufacturer’s instructions concerning drying times should be followed, and coatings must be fully dry before the application of subsequent coats.

Before applying new coatings to painted stainless steel, the original coating system should be identified and advice on surface preparation obtained from the paint system manufacturer. This should help ensure compatibility between the new and existing coating systems.

Paints that contain metallic zinc should not be used on stainless steel as embrittlement of the stainless steel substrate can occur in the event of severe fire damage.

Before commencing any task ensure that you have received the appropriate health and safety literature from the supplier and fully understand it. If in doubt seek advice

In the last revisions of the various parts of BS EN ISO 12944, class C5I, (where ‘I’ signified Industrial), and class C5M, (where ‘M’  signified Marine), were merged to give a single C5 class of very high corrosivity, but C5 is now exclusively for land based structures. For offshore structures there is the new CX class for conditions of extreme corrosivity. There is also a new immersion class, IM 4 for sea or brackish water where cathodic protection is employed, (IM2 is for sea or brackish water, but without cathodic protection). Any paint system for the new CX extreme corrosivity class must be able to pass the new cyclic testing regime which is described in BS EN ISO 12944-9:2018.

We also find that the minimum specified coating thicknesses, NDFT, for this new corrosion class are greater, as shown in the table below, where all the Zinc based primers featuring in Table 3 of part 9 of the standard have been omitted here, for the reason stated above.

Minimum requirements for protective paint systems and their initial performance

Corrosivity Class of Environment CX (offshore) Splash and tidal zones CX (offshore) and IM4 Im4
Primer NDFTa (microns) >/= 60 >/= 60 >/= 200 >/= 150
Minimum number of coatsb 3 3 2 1 2
NDTF of paint system (microns) >/= 350 >/= 450 >/= 600 >/= 800 >/= 350
Minimum pull-off test value (before ageing) determined in accordance with ISO 4624, Method A or Bc (MPa)d 5 5 5 8 5
a NDFT – the nominal dry film thickness

b The number of coats does not include a tie coat, which might be needed under certain circumstances.

c It is required that the force built up is controlled and linear as described in ISO 4624, e.g. by using an automatic hydraulic test equipment.

d Push-off adhesion testing is not permitted.

 

Typically, paint systems suitable for the CX environment would be expected to be similar to those given above for the C5 Class, (and previous C5I and C5M corrosivity classes), but with the thickness of the primer coat increased to a minimum thickness of 60 microns, and with the thickness of the subsequent coats increased to give at least the minimum overall values indicated in the above table. However, the more rigorous testing/validation of their performance and categorisation into the ‘new’ durability classes might impact how paint systems are marketed regarding their suitability for the different environments/corrosion classes.  Manufacturers’ instructions should always be followed and adherence to the various parts of BS EN ISO 12944 is obligatory in order to achieve the required performance. It should also be confirmed with the paint system manufacturers that the system is suitable for use on stainless steels and is suitable for the CX extreme environment, and thus has been validated to the various parts of BS EN ISO 12944, especially BS EN ISO 12944-9:2018.

 

 

← Back to previous

↑ Top