CoatingsPro Magazine

JAN 2018

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Page 22 of 116

22 JANUARY 2018 COATINGSPROMAG.COM have Googled extensively and can't find anything that indicates this is a recog- nized practice. A: I have to ask the question: W hy do they want to put aluminum over a zinc coating? You can get a zinc-aluminum wire of 85 percent/15 percent mix. It's usually one or the other. If they want greater protection, you just apply a thicker coat. We do a lot of thermal spray, and it is usually zinc or aluminum, or a mix of 85 percent/15 percent. A: One is a "cladding," and the other is an alloy. I don't know if this makes a difference. A: A clad material could be an alloy. TSA is thermally sprayed alumi- num, where aluminum powder or wire is brought to the melting stage. Ionized particles are forced through a nozzle or nozzles to the target substrate, and a uniform coating is achieved. In this case, the applied aluminum is an element. Claddings also are of different types. For example, they could be mechan- ically clad or metallurgically clad. In mechanical clad, the substrate and clad alloy are not fully bonded. Some pipes are clad by this process. A metallurgical cladding achieves full bonding by fusion. In the metallurgical group, there are also various types, like cladding by welding for a relatively smaller surface area. Explosion clad is often used for larger surface areas, like plates. A: It seems to me the question here is not if TSA is okay over galva- nized steel, but which kind of TSA. A: In respect to coatings, consider that both zinc and aluminum are metals used for their sacrificial properties when coupled with more noble materials — and the fact that both are very close together on the less noble (anodic) end in respect to the galvanic series. I don't see a significant issue with combining these two metals at any percentage, be it 85 percent/15 percent, or otherwise. Both metals will sacrificially protect the more noble substrate. ere is some potential for galvanic corrosion to occur between the two, with aluminum being the more noble metal. But it is probably negligi- ble. A lthough I do agree with verifying that the substrate profile must exist per the specifications. A: I have done TSA over a bare metal surface, but have never heard of TSA over galvanized steel. In my opinion, if TSA is to be applied, galva- nizing is not required. Pitting Issue After Blasting, Shop Priming Q: In the ship building sector, we often bring plates and pipes from suppliers who obtain their materials from mills around the world. Depending on how well the suppli- ers store and how long they store the materials before delivering to us, these plates come in rust grades A, B, C, and D, according to ISO 8501-1:2007 and reference from SSPC (Society for Protective Coatings) VIS (visual reference) 1 (Guide and Reference Photographs for Steel Surfaces Prepared by Dry Abrasive Blast Cleaning). However, most of the time, we also face situations such as a single plate. ere are a combination of rust grades because of localized corrosion taking place. My first question is this: Are we able to accurately predict whether the plates have pits before sending it for blasting and shop priming at its initial condition? Have you had incidents where the plates looked like rust grade C but after blasting the pits were gone? And with rust grade B after blasting, are there pits present? Does using sandpaper (grade 60) to abrade it work? One reason could be that I misinterpreted the plates as grade C simply because rust corrosion, which resembles pitting, occurs only on the mill scale but not on the substrate. Another reason could be crevice corro- sion, which already occurs beneath the mill scale deemed grade A. In the IACS (International Association of Classification Society) shipbuilding and repair quality standard, there are acceptance criteria of pits allowed without remedy. Beyond that, defects can be resolved by grinding or welding in accordance with the IACS standard. In one case, the client rejected the plates with the slightest pitting, expecting no pits to be present during material inspection. Corrosion takes place, and the plates are transported around the world. We could only reduce the rate of corrosion by having better packaging and a sheltered storage area. Is it justifiable for the client to completely reject the plates, or is the client expecting too much? A: Please check your specification to see if it mentions the rust grade of material prior to blasting or painting. e second thing is, go to an engineer to check after grinding and see what minimum thickness is accept- able, as per material specifications. A: is pitting-related question may go better through a classi- fication society. I recall some similar issues from shipyards in 2009 when the ship building industry had slowed down. We have a very similar situation now. e shipyard blames steel mills for pitted products, and ship owners blame shipyards for pitted plates on their ships. Class is in the middle of their argument. New buildings normally use steel rust grade A or B (new steels). Grade C steel has light pits, which can be easily cleaned by blasting. However, Grade D steel could be heavily pitted and may not easily be cleaned by blast- ing to [ISO standard 8501-1] Sa 2½, which is normally required for the agreed specification. Pitted steel in the shipyard is ty pically caused by an inappropriate outdoor storage of steel plates. In that case, heav y pits around overlapped edges of steel plates can be expected. I ACS and classification societies have repair requirements on the pitted plates. Classification sur veyors should know all details of the requirement. Some rough surfaces may be caused by Notes From the Blog

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