wide beam 'Daedalus'

[ChrisPy]

There was at least one horror story on this forum about a 3 or 4 year-old boat with the paint falling off, by which time it was out of warranty. But I'm sure Phil's experience is second to none and probably represents a reasonable approach to the risk.

 

Pity there isn't more literature available about the possibilities of painting over 'sound' millscale.

[Gary Peacock]

Pickled and oiled steel is going to add maybe £500.00 to the price of a boat it does remove the mill scale totally but you still have the exspense of keying the bright steel by a mechanical means too. Strictly speaking all steel should be prepared prior to fabrication and welding which in effect pickled steel is.

 

This is quite a chunk from the CBA Corrosion Control report that was prepared by some of the accepted experts on the subject, it makes some quite interesting points but gives some hints to why things can get a bit "iffy" where steel is concerned. (Sorry you can't have the full report it is Copyrighted and not for the publics eyes without you part with £75.00 I wonder why! )

 

 

 

Stage 1 Steel Selection

Selection of appropriate steel for building is the prime concern of the boatbuilder. The steel provides the very fabric of the boat and is the focus of attention of this report on corrosion prevention. From a review of the technical papers given by Corus, it will be seen that selection of the correct and most suitable steel is not as clear cut as it may seem.

The availability of a wide variety of qualities of steel is well known, but what has been confirmed by Corus is the need to understand the differences in types and classification of steel. The long held belief that the formulation and quality of steel depended on its geographical source has not been refuted although in answer to direct questioning it was clear that western steel, ie that produced in western Europe or the US, is preferable to that produced in the Indian sub-continent or similar regions. There is of course a significant pricing differential between western and non-western steel and this must be considered by the boatbuilder in ensuring the requirements of the customer with regard to steel quality and corrosion control are taken into account when ordering plate.

Referring to Corus Structural steel plate grades and standards, the current European standard for the specification of hot-rolled non-alloy steel properties for section and plate products is noted as EN 10025. This standard was published by British Standards (BSI) in 1993 and superseded BS 4360:1986 which has been declared obsolete. This means that steel should be ordered against BS

EN 10025, with the grade S275 representing that formerly known as 43A. Steel stockists may still accept orders against the old standard but it is important to understand the requirements of the current BS EN 10025 and the limitations it places on steel graded under this standard.

BS EN 10025 calls up two further European standards which specify the dimensional and shape tolerances for hot rolled steel plate and continuously hot rolled steel plate, EN 10029 and EN 10051 respectively. The standards were published by BSI as BS EN 10029 in 1991 and BS EN 10051 in 1992. Careful attention should be paid to the tolerances allowed under each of these standard, particularly that relating to thickness.

Referring to Corus Structural steel plate grades and standards, it will be noted that thickness tolerances vary considerably against class of steel under BS EN 10029, hot rolled steel plate. Class A defines tolerances for structural steel plate and Class B for ship plate and pressure vessel steel plate: these two classes of steel would be the most commonly supplied class of steel to canal boatbuilders. Steel plate to either class A or class B thickness tolerance under BS EN 10029 could have a plate thickness of up to -10% on the nominal gauge, ie a 4mm plate could be as little as 3.6mm thick under class A. On 10mm plate this tolerance reduces to 5%, but this is still a significant differential over the nominal plate thickness. Plate to class D could be as much as 15% down on thickness on thinner plates. The converse is that in all classes the over-gauge tolerance can be as much as 20%.

The working group considered the understanding of the specification, grade and class of steel plate to be fundamental to the work of a steel inland boatbuilder and believe that awareness of the potential for plate to be considerably thinner when delivered from the stockist than that originally envisaged is an important first lesson to be learned from the Corrosion Control report. In many of the instances of corrosion on steel inland boats, the evidence given in support of a problem is reduced plate thickness. It may be that in many cases the original plate was already some way off that declared as nominal thickness and this information must be taken into account when investigating corrosion.

The remainder of the Corus Structural steel plate grades and standards paper presents much useful information on length and width tolerances and gives an indication of what to expect in way of material properties of steel plate ordered to BS EN 10025. The option for CBA to develop an industry-specific plate procurement standard is suggested and this is a subject for the CBA Committee to review.

 

 

Stage 2 Steel storage and handling

On delivery from the steel stockist, steel plate is usually not destined for immediate use by the boatbuilder. It is stored, often for some considerable time due to the stockist’s minimum ordering quantities, and plate is drawn from the boatbuilder’s store on a project building basis. The processes involved in receiving steel plate from the stockist, storing and then working all necessitate a significant amount of handling of the plate, whether by forklift, mechanical jacks or other handling tools. The storage and handling of steel plate are, after selection of the plate from the stockist, two of the most important aspects of corrosion control when building steel inland boats and particular attention should be paid to issues raised in the second and third technical papers from Corus, Corrosion protection of steel structures and Defects in reversing mill plates.

It is essential that steel plate stored at the boatbuilding site is suitably protected from environmental degradation. Steel plate delivered from the stockist should ideally be stored in a weather-proof store that is regulated for moisture differentials. Storage outside in an unprotected store will give rise to excessive corrosion of the plate before it has even begun to be worked. A damp or wet environment is extremely damaging to untreated steel plate and layers of rust will form that may start to corrode the plate in-situ, attacking defects such as scratches, cracking, pitting and similar indentations.

The corrosion of steel plate is also affected by the presence of contaminants, whether in the plate itself or deposited on the surface during storage and working. The cleanliness of the store is therefore important, as is the removal of any residue on the plate on receipt from the stockist. Steel plate may be ordered ready primed and the corrosion effects of contaminants during storage will be limited, however proper storage conditions should be adhered to for all specifications and a clean and dry store location is the prime factor in controlling corrosive effects during on-site storage.

Handling of the steel plate while on-site is equally important and care must be taken to protect both the plate and the machinery and personnel undertaking handling activities. Damage to steel plate while lifting with forklifts can cause significant corrosion problems as indents will harbour wetness and contaminants, cracking will scar the surface, and scratches will break the surface integrity of the plate, all of which will leave the plate more susceptible to localised corrosion. Use should always be made of appropriate handling equipment and implementation of best safe practices will protect both the materials and personnel.

Stage 3 Steel preparation

Preparation of steel plate in readiness for application of protective coatings presents possibly the first compromise that the boatbuilder has to consider in building the boat. If the plate surface preparation is not correct, the best coating in the world will not protect the steel, however, the best systems for preparation also present the most significant health and safety and environmental protection issues. Both Corus and International agreed in their presentations that blasting steel plate was the most effective and successful way of preparing plate for coating, however the reality of the use of such techniques is somewhat less than ideal for the boatbuilder who may be faced with constraints effectively outside their direct control depending on location and facilities available.

Surface preparation is required primarily to remove mill scale and to provide a key for paint systems to adhere to the plate. Surface preparation also removes other contaminants such as dust, flash rusting, oils and grease, effectively providing a bare plate for application of coatings. Failure to remove mill scale and other contaminants will result in paint systems failing with the result being patches of coating falling off the plate and leaving it exposed to further degradation. As noted above, the rate of corrosion of steel plate is dependent on a number of factors, one of which is the presence of contaminants. To provide a good base for coatings and for long-lasting protection, suitable and adequate surface preparation is key to the process.

There are two basic methods of surface preparation: mechanical cleaning and blasting. Mechanical cleaning includes electric wire brushing, power sanding or disking and flame cleaning, each of which is suitable for preparation of steel during maintenance but less so for new work. Effectiveness rates vary from 25% to 35%. Blasting encompasses dry and wet abrasive blasting and high-pressure water jet blasting, and may be done on uncut plate or after construction. Each of these methods is suitable for maintenance or new work and each, when performed correctly, is 100% effective at removing surface contaminants and mill scale. The plate will be left bare and will be provided with the best surface for application of coatings.

Acid pickling is also a method available for surface preparation, and although again extremely effective, its use is confined to new work only. While pickling steel plate is not recommended by International, the working group consider that it does have a place in the inland boatbuilding industry when used with phosphoric acid etch primer. Etch primers similarly are accepted by the working group as being suitable for inland steel boat building.

The Corus technical paper Corrosion Protection of Steel Structures gives more information on surface preparation techniques and provides an explanation of preparation standards and visual inspection methods.

Blasting is considered the most effective technique for preparation of steel plate, giving the most resource-efficient and successful means of ensuring full coverage of the plate and can be done before the build as pre-blasting or after the build. Corus and International recommend grit blasting to SA2.5 standard (BS 7079-A1 equivalent to ISO 8501-1 Preparation of steel substrates before application of paints and related products), providing a surface that is suitable for accepting protective coatings. Such grit blasting is carried out in a wide range of industries, but the product concerned is often a high-value or key infrastructure element that is required to be minimum-maintenance such as steel buildings, bridge girders or ship hulls and superstructure. The working group considered that achieving SA2.5 standard was unnecessary for inland steel boat construction but that care should be taken to ensure an adequate profile was achieved.

However, the health and safety problems encountered by the operator of the blasting equipment when working in confined spaces and the environmental pollution from blasting residue means that grit blasting is considered to be uncommon as a means of preparing steel plate for coating in the inland boatbuilding industry. Facilities for such works do exist but the associated costs makes the final cost of construction significantly more expensive. Additional problems with blasting include possible deformation of thin plates: rippling of plates less than 5mm or 6mm thick is a real possibility and great skill and care must be exercised in using such techniques when working on plate areas such as the superstructure.

Mechanical cleaning provides a cheaper and more practicable method of surface preparation with less onerous health and safety implications than blasting. Disking and wire brushing is the most common method of surface preparation in the industry and is recommended by the working group as the preferred method of surface preparation in the majority of new builds.

This approach has been taken with due consideration of the relative effectiveness of different systems and the health and safety aspects of each, with an overarching constraint being the cost of blasting systems relative to the final value of the product, as determined by the customer. A builder may consider offering the customer the option to have the steel blasted, but this must consequently be reflected in the price and delivery timescale if outside resources are required to undertake such work.