Question:
Under what types of exterior atmospheric exposure (other than marine) is it necessary to test for soluble salts before maintenance painting? PS
Answer:
The effect of soluble salt contamination on protective paint systems is well documented in terms of the acceleration of electrochemical corrosion processes and the propagation of osmotic blistering of paint films that are applied over salt contaminated substrates.
Paint specifications will normally define maximum levels for soluble salt contamination on a substrate. These can sometimes be set by an external specifying body (e.g., owner specifications, NORSOK, Network Rail, Highways England), or by the paint manufacturer.
Whilst the presence of a high level of salt on a surface might be obvious in a marine location due to the high levels of chloride in the atmosphere caused by saltwater spray, it is quite reasonable to expect salt contamination at an inland location. Industrial pollution is much reduced nowadays, due to tightening of air quality regulations, however considerable amounts of nitrates and sulphates are present in road traffic exhaust, and in addition large amounts of road de-icing salt are deposited throughout the winter months on the roads of all countries where winter freezing conditions may be expected. This salt is whipped up into a concentrated salt spray by the action of moving traffic on the wet roads which will deposit a persistent salt contamination that can spread some distance from the road on which it was deposited.
The simple answer to the question would therefore be that it is good painting practice to assess the surface salt level on any substrate that is to be painted, whether it be in a marine or inland location.
If the measured salt levels are low and meet the specification, then the project may proceed with occasional salt testing to ensure that the salt levels are still within specification. If local salt levels are high and exceed the specification, then the substrate will require fresh water washing and re-checking of the salt levels until the specification is achieved. In these contaminated locations then a more rigorous programme of washing down and salt level assessment must be agreed between the asset owner, painting contractor, paint manufacturer and independent inspectors.
Surface cleanliness is defined under the ISO 8502 series of international standards, along with equivalent standards from organisations such as NACE and SSPC.
ISO 8502 consists of the following parts, under the general title:
“Preparation of steel substrates before application of paints and related products — Tests for the assessment of surface cleanliness”.
— Part 2: Laboratory determination of chloride on cleaned surfaces
— Part 3: Assessment of dust on steel surfaces prepared for painting (pressure-sensitive tape method)
— Part 4: Guidance on the estimation of the probability of condensation prior to paint application
— Part 5: Measurement of chloride on steel surfaces prepared for painting (ion detection tube method)
— Part 6: Extraction of soluble contaminants for analysis — The Bresle method
— Part 9: Field method for the conductometric determination of water-soluble salts
— Part 11: Field method for the turbidimetric determination of water-soluble sulphate
— Part 12: Field method for the titrimetric determination of water-soluble ferrous ions
In addition, new ISO standards are under development to describe some of the other commonly used methods for determination of soluble salt levels.
Malcolm Morris
