Ask the Expert

This month, the questions being answered by our corrosion technology experts relate to impressed current Cathodic Protection systems for pipelines and plant piping, and salt contamination of metal surfaces before painting.

The use of linear MMO sock anode systems are specified by various operating companies for pipelines and plant piping cathodic protection systems. The specifications also state that an effective isolation is not compulsory in a congested petrochemical plant, as the anode current is expected to protect the pipeline closer to the anodes. Should effective isolation be compulsory. If not, what other protective measures should be taken? AN

Distributed anode cathodic protection systems are generally used for plant piping protection. Achieving 100% isolation on a complex structure / plant piping is practically difficult which leads to huge current loss to other structures, limiting the cathodic protection on piping. The project specifications developed in the last decade specified linear MMO sock anodes for complex structures to overcome the current drains due to isolation failures. The basic assumption being that the anodes are installed closer to the pipe and hence the CP current will be drained by the pipes due to proximity rather than the earthing electrodes, or concrete rebars, a few metres away from the anode and the pipe. The same concept has been applied on several plant piping and pipeline projects, and current drains to earthing rods and concrete rebars are evident even with a sock anode system. The cathodic protection levels on the piping improved after fixing the failed isolations. It is known that an effective isolation is the key to achieve cathodic protection of pipes irrespective of the type of anodes being used.
It has also been observed that IR free coupons installed in the vicinity of sock anodes are influenced by the anodic current during IR free / Instant OFF potential measurements. This is an additional information for the CP designer to consider while designing a sock anode system. The coupons that are very close to the anode polarise positve and affect the pipeline polarisation when connected to pipes. Coupon potentials turn more positive during Instant OFF as it interferes with the anode. IR free coupons and reference electrodes must be placed away from the sock anodes, or the use of IR free coupons must be avoided on a sock anode system to avoid detrimental effects.
Ashokan Gopal, Corrosion Technology Services Europe Limited.

What is an acceptable level of salt contamination on a surface before applying a protective coating? Does this level vary with the type of coating applied, or the end use? JW

What at first seems a simple and straightforward question is not a simple and straightforward answer! The issue of salt contamination has always raised a heated debate since the late 80s early 90s when people started to realise that residual salts were a main cause of premature breakdown of coatings. Prior to this the most likely level of testing was the use of a potassium ferricyanide test paper that was applied to areas of pitting, to see if there was any residual ferrous salts present. A qualitative, not a quantitative test so we only know if ferrous salts are present or not, not how much, so is of limited value. Since then, a whole gamut of possible salts has been recognised as potentially being present on the surface of prepared steel, still the ferrous salts, but most importantly the presence of sodium chloride (NaCl). NaCl has always been known to be a problem for coatings, it took, however, a while for people to realise that the risk was from osmosis, and the creation of osmotic blisters containing a strong saline electrolyte.

Osmosis is the process whereby two solutions on either side of a semi-permeable membrane try to reach a state of equilibrium regarding their concentration. If you have a strong solution on one side and a weak solution on the other, water will pass through the membrane attempting to dilute the strong solution until it is isotonic with the other side of the membrane. From this we can therefore deduce that salts become a major issue in either immersion or very damp conditions, blistering is not going to be very likely in a dry, air-conditioned environment. The next thing to consider is the permeability of the applied coating, this will be affected by a multitude of factors but mainly the density of the coating matrix and its resistance to the flow of water through it, and the applied thickness. Also, the concentration gradient across the membrane must be considered.

When it comes to coatings, there are several considerations, particularly when using certain words; glass flake is a particular one when it comes to permeability! Glass flake coatings come in a whole range of varieties of binder, flake size, flake shape and flake density. A cheaply made ground glass powder in a cheap epoxy binder applied at low DFT will never perform as well as a proper, high density glass flake with a flake size of 1/16” or larger, trowel-applied, solvent free polyester based coating, with a DFT of over 1mm!

There is then the issue that stainless steels, duplex and super duplex materials suffer from chloride induced stress corrosion cracking and with these, when in a high-risk application, there is a need to see chloride ion contamination as close to zero as is practicable. This is not helped by the fact that many epoxies have chlorides in their formulations, so regardless of how low you get the contamination on the surface, the wrong selection of coating immediately undoes all the hard work!

As can be seen, there is no one answer fits all, unless you take a totally risk aversed viewpoint, where there is a zero tolerance.

There are two ‘normally acceptable’ values that tend to be bandied around and these came originally from the NORSOK M-501 guidelines:
“The maximum content of soluble impurities on the blasted surface as sampled using ISO 8502-6 and distilled water, shall not exceed a conductivity measured in accordance with ISO 8502-9 corresponding to a NaCl content of 20 mg/m² .”

20mg/m² was adopted for immersion and 50mg/m² for atmospheric maintenance. These numbers are not based on any particular science but are pretty arbitrary and were there as a guideline. The next problem is the interpretation – is it total salts? a specific salt such as sodium chloride? or is it just the chloride part? Having defined what you deem necessary to test for, the next problem is how do you test for it? This opens another major can of worms!

In all honesty, there is not a single answer, you need to look at the individual requirement. The coating manufacturers should have done the necessary testing in a variety of scenarios with their products, and should be able to confirm what level of contamination is acceptable and what DFT of the material is required to give the required performance in the situation envisaged. Independent verification of the testing by a third-party test lab is a very useful indicator of the potential performance to confirm manufacturers’ claims.

Therefore the simple answer is to use the 20 and 50 mg/m² for carbon steels etc. and as close to zero for S/S, duplex and super duplex as a safe bet base guideline to work from, but it is essential to make sure that the materials, specification and environment are assessed together with making sure that the most suitable choices are made to meet the performance needs.
Simon Hope, Consultant Technical Authority, Auquharney Associates Ltd.

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