By: Dr. Andrew J Pinkerton, The University of Manchester

The environment continuously attacks our infrastructure in a way that is less spectacular than, for example, earthquakes or hurricanes, but far more expensive. Corrosion affects most surfaces, but especially metallic ones, on a daily basis and costs around 3–6% of developed countries’ GDP to combat. In the US alone corrosion costs are an estimated $276 bn/year, compared to the average annual cost of around $13 billion due to hurricanes. [1,2]

Corrosion can be combatted, however this can be expensive and it is necessary to match the method to the surface for it to be cost effective. For larger, low-cost parts such as pipes and bridges, painting, and if necessary repainting when necessary, remains popular. For higher value parts, such as car panels or chassis, zinc coating applied via electroplating is a common method. For the highest value parts, for example in the aerospace industry, a corrosion-resistant material may be used for the complete component.

Laser Cladding is a method of surface coating by powder fusion that is used when high performance, wear or high temperature corrosion resistance is required (aerospace engines, power stations etc). It traditionally uses spherical metal particles of smaller than 100 um diameter as the raw material for the coating. During the cladding process these are propelled into a melt pool created by a laser moving across the surface of the material to be protected. When this melt pool solidifies it forms a solid layer fused to the original surface and this then acts as a passive barrier to corrosion. A typical protective clad layer is shown in Figure 1. [3]

Typically, laser cladding is relatively expensive method, partly due to the capital cost of the equipment but mainly due to the cost of the spherical, gas-atomised powder. However, recent work has shown that it is feasible to use prepared machining swarf instead of this powder without compromising the corrosion-resistance of the final layer. This reduces material costs to close to zero, and potentially dramatically increases the range of application of the method. Figure 2 compares the powder that has typically been used and the new prepared swarf. [4]

 

 

 

The reduction in corrosion that can be provided by a clad swarf layer is dramatic. For example, corrosion tests in a NaCl solution reveal that the corrosion rate of uncoated mild steel was 34.7 mm/year, while that of the same samples laser clad with Inconel 617 swarf was less than 0.1 mm/year [5]. Using greater laser energy per unit area during the deposition of the protective layer further enhances the corrosion resistance, probably because it leads to a coarsening of microstructure, reducing the number of grain boundaries (Figure 3).

So, the combination of the reducing capital cost of lasers and now low material costs make laser cladding a weapon that could be used more widely in the battle against corrosion. It remains only one of many methods, but when faced with an annual bill of $276 bn/year any improvement offers significant cost savings.

 

References

[1] FHWA (Federal Highway Administration) report FHWA-RD-01-156, FHWA, Washington USA, 2002

[2] The Cost of Climate Change – What We’ll Pay if Global Warming Continues Unchecked, F Ackerman, E A. Stanton et al, NRDC (Natural Resources Defense Council), NY USA, 2008

[3] K. Mahmood, N. Stevens, W U H Syed, A. J. Pinkerton, Material-efficient cladding for corrosion resistance, 30th International Congress on Applications of Lasers and Electro-Optics, ICALEO 2011 Congress Proceedings (Laser Institute of America, 2011).

[4] K. Mahmood, W U H Syed, A. J. Pinkerton, Innovative reconsolidation of carbon steel machining swarf by laser metal deposition, Optics and Lasers in Engineering 49(2), 2011, p240-247.

[5] K. Mahmood, N. Stevens, A. J. Pinkerton Laser Surface Modification by a Unique Form of Coating Material, Journal of Materials Processing Technology, submitted

List of Figures

Figure 1. Cross-section through a laser clad layer (A environment, B clad layer, C substrate)

Figure 2. Laser cladding build materials (a) traditional spherical powder, (b) prepared machining swarf

Figure 3. Microstructure scale and Corrosion rate of laser clad surfaces v energy per unit area used to deposit them (corrosion rate for uncoated layer = 34.7 mm/year)