Wet Abrasive Blasting for Marine Applications
Mild steel is the most popular metal for constructional purposes due to its relatively low cost, mechanical strength and ease of fabrication. Its main drawback is that it corrodes easily in seawater and unless adequately protected, rapidly loses strength which may result in structural failure. Seawater is an aggressive environment for marine vessels, steel infrastructure, offshore platforms, pipelines and tanks.
Galvanic corrosion, stray current corrosion, crevice corrosion and microbial corrosion are all different types of corrosion that can affect metals in marine environments. Metallic corrosion leads to increased structural failures and huge economic losses in various industries. Corrosion comes with a tremendous cost, and the reminders of this are everywhere: burst oil pipelines, collapsed bridges, cargo-laden ships at the bottom of the sea etc.
All steel structures such as cranes, masts, or containers must be protected against corrosion and environmental influences. Marine vessels must be restored and maintained for them to carry out their transport function. Repairing the crumbling infrastructure is one of the government’s most significant challenges and significant funds are allocated for it in the budget. Private and pubic companies also recognise the importance of maintaining their assets.
Abrasive Blasting is Key to Effective Protection Against Corrosion
Surface preparation is very important for any sort of corrosion protection system to operate effectively. All forms of contaminants need to be removed before applying protective coatings in order for them to adhere to a metal or steel surface properly and to be effective in keeping rust and other types of corrosion at bay.
Not only does the surface just have to be clean, it also needs to be profiled in such a way that the coating can adhere to it effectively. Abrasive blasting is a flexible and precise approach to surface preparation that enables metal, steel and other surfaces to be profiled to meet precise specifications and requirements.
There are two main forms of abrasive blasting – Dry Sandblasting and Wet Abrasive Blasting that are used to remove corrosion.
Why is Wet Abrasive Blasting more effective at profiling and surface preparation than Dry Sandblasting?
Dry sandblasting involves propelling dry abrasive with compressed air onto the substrate to remove corrosion. While, the process is faster, a significant disadvantage of dry blasting is that it can in fact drive contaminants such as oil and grease into the substrate, rather than clean them off. Additionally, parts of the sandblasting media itself can become embedded in the substrate if blasting is done without the cushioning effect that water provides.
Using water in the wet blasting process means that the blast grit does not break down in the same way as in dry blasting. and so significantly less is used, reducing costs, both in terms of materials and cleaning up after a job is completed. (It should also be noted that wet abrasive blasting uses far less water than UHP cleaning, because the water itself is not being used to remove contaminants, and so less water pressure is needed.)
The use of water in the wet abrasive blasting process results in virtually no dust being generated as it is encapsulated in the water molecules. This is very beneficial as there is very little dust clinging to the surface after blasting and does not negatively impact the adhesiveness of the protective coating applied. Dry blasting or sandblasting does not remove dust as effectively as wet blasting, which can make the whole process more time consuming and expensive as a further clean (usually either by brushing or vacuuming) has to be undertaken after the blasting process has been completed.
Lastly, environmental regulations require that dry blasting must be performed in a contained area such as a blast room. Some of the marine structures and vessels are difficult to be transported or encapsulated. This is where mobile wet blasting is extremely useful as the surface does not need to be encapsulated as there is virtually no dust generated. Wet abrasive blasting can be done in situ and there is no need to transport the surface. Dustless blasting machines such as the RapidBlast™ abrasive blasting equipment are extremely portable and can be used on bridges, tanks, large ships etc that cannot be transported.
Prevent Flash Rusting with Wet Abrasive Blasting
Flash rusting can present a significant problem when preparing metal structures for coating and painting, as it forms very quickly on steel that has been newly blasted, especially when sandblasting or other conventional dry blasting are methods are used. If not checked and controlled, flash rusting can fatally undermine the integrity of steel marine structures, boats, ships and machinery and cause protective coatings to fail.
Flash rust is caused by salt deposits on the surface of newly-blasted metal (a constant hazard in marine environments) which, when mixed with water, cause corrosion cells to form. The problem is exacerbated and accelerated in humid environments.
In dry blasting, the sandblasting media is not rinsed off leaving soluble salts and other contaminants on the surface to a certain degree.
In wet abrasive blasting, a slurry of water and sandblasting media is propelled at high pressure onto the surface. A combination of the blast media cushioned by water removes the coating/corrosion but soluble salts are not left on the blasted surface as the water dissolves them and then washes the salt away.
The use of a rust inhibitor such as RapidHold™ SuperStrength provides additional protection against flash rusting for upto 72 hours. The rust inhibitor can be added to the slurry mix of water and sandblasting media while blasted. The RapidBlast™ wet blasting machine also has an option for wash down whereby the surface can be rinsed off using the solution of the rust inhibitor mixed with water. The RapidBlast™ abrasive blasting equipment also has an option of remote washdown whereby the washdown can be activated and controlled from the deadman handle itself without having to go back to the control box on the dustless blasting machine to change the controls.
Best Practice
For the best rust-prevention and coating adhesion results, follow these best practices:
- Blasting should impart a proper anchor pattern.
- Leave the surface as clean as possible free of salts.
- Blast in low-humidity conditions.
- Use a Rust Inhibitor
- Apply coating as soon as possible.
To summarise:
Corrosion and foul build-up is a massive problem for marine vessels and steel infrastructure. Surface preparation is the key for any sort of corrosion protection system to operate effectively. Dry blasting as well as wet blasting are effective but wet blasting provides significant advantages over dry blasting such as removal of salts, portability of abrasive blasting equipment and dust suppression. It provides a growth opportunity for industrial sandblasters with mobile abrasive blasting equipment to prepare surfaces for corrosion prevention. The use of a rust inhibitor provides additional protection against flash rusting until a protective coating can be applied.
