PIT | Monitor and forecast pitting corrosion in maritime steel structures

Mechanisms and Impact of PITting on Carbon steel structures in a maritime environment

The Blue Economy is a crucial part of the Flemish economic fabric. It relies on extensive harbour, coastal, and offshore steel infrastructure. Additionally, offshore wind turbines are among the most powerful and sustainable sources of green energy today, playing a key role in combating climate change.

On the Belgian coast, several wind turbines, built in 2009, were designed to last 20-25 years. This region also hosts some of Europe’s most important harbours. While corrosion management strategies such as corrosion allowance, coatings, and cathodic corrosion protection help mitigate uniform corrosion, the risk of pitting corrosion remains understudied.

Pitting corrosion creates small, localized holes in metal surfaces, which can serve as crack initiation sites, potentially leading to a significant reduction in the fatigue lifetime of steel structures. Currently, these structures are visually inspected, requiring removal of corrosion products and macrofouling. This process is time-consuming, error-prone, and inefficient. But what if we could detect pitting corrosion earlier, more effectively, and with less effort?

Context 

Operators and asset owners suspect that pitting corrosion significantly impacts the lifetime of offshore and maritime structures. However, the extent of this risk remains uncertain, leading to gaps in the design calculations.

Currently, pitting effects are only indirectly considered through S-N curves for free corrosion. This is due to the unclear nature of corrosion mechanisms in a maritime environment:

• Does carbon steel undergo true pitting corrosion, or is it heterogeneous uniform corrosion?
• Under what circumstances does pitting develop?
• How fast do pits grow, and what are their shapes, sizes, and distribution?
• How does pitting affect the (fatigue) lifetime of steel structures?

Moreover, there are currently no proven methods to monitor the occurrence and growth of corrosion pits in-situ.

Objective and results

The essence of the intended scientific innovation in the PIT project, is to gain an improved fundamental understanding of how and under what circumstances corrosion of carbon steel in a maritime environment can lead to ‘pitting’, where here, a ‘pit’ is defined as a localized attack leading to a reduction of the lifetime of a structure. In addition, we want to obtain quantitative data on the size, shape and growth rate of 'pits’ by: 

1. developing new, robust, operator independent and standardisable methods to analyse corrosion coupons;
2. research methods to continuously monitor pitting susceptibility of carbon steel using electrochemical techniques.

Finally, a method needs to be developed to translate the obtained quantitative data into input for existing lifetime models. 

The PIT project does not develop new fatigue lifetime models, but it investigates how to improve existing models with better corrosion data.

The project focusses on the Offshore Renewables (fixed foundations) and maritime sector, although cross-domain valorisation can be expected to floating energy structures, energy islands, mooring systems, shipping, aquaculture, freshwater production, the chemical industry and wastewater treatment. 

Key Exploitable Results (KERs) 

The project will deliver 4 Key Exploitable Results for industry application:

• KER1: Understand under what circumstances carbon steel corrosion can lead to ‘pits’; and what are the influencing factors, the impact of Cathodic Protection and locations at highest risk.
  Valorisation objective: Support decision making with respect to both the design and O&M of assets.
• KER2: Understanding of electrochemical measurement principles for pitting of carbon steel.
  Valorisation objective: Sensor development, and real-time corrosion monitoring (OPEX reduction).
• KER3: Method to obtain quantitative data on the size, shape, distribution and growth rate of pits, from analysis of corrosion coupons and electrochemical measurements.
  Valorisation objective: More reliable tools for lifetime (re)assessment (reductions of LCoE).
• KER4: Large, unique, dataset on evolution of pitting corrosion of carbon steel in maritime environments.
  Valorisation objective: Improved future designs (CAPEX reduction); validate/improve corrosion models.

Approach

The PIT project follows four main research tracks:

1. Investigate the use of electrochemical techniques for continuous monitoring of pitting corrosion:
   1. The work will start in the laboratory by simulating conditions under which pitting corrosion can occur. Different electrochemical techniques (ECN, EFM, EIS) will be investigated for their feasibility to detect pitting.
   2. The most promising techniques will be trialled in the field at a test setup in the Harbour of Ostend.
2. Develop a method for automated analysis of corrosion coupons:
   1. Corrosion coupons (small metal plates exposed to the same environment as the structure being studied) are typically manually investigated for the presence of pits.
   2. Within the PIT project, a method to automate the analysis and make it operator independent and objective will be developed. A large number of pitted carbon steel coupons from different exposure sites and duration are available at the project partners to kick-start this development.
3. Investigate the impact of macrofouling on pitting corrosion of carbon steel:
   1. In the harbour of Ostend a floating pontoon will be used as a test-bed to not only test different monitoring solutions, but also investigate the type and amount of macrofouling that develops and how that impacts to occurrence of pit corrosion.
   2. Analysis of the local environment under the macrofouling will be done using microprobes to understand how different types of macrofouling affect the surface.
4. Catalogue the type of input data existing fatigue models require:  
   1. Although the PIT project does not aim to develop new fatigue models, we want to understand what models exist and what type of input data with respect to the geometry and extent of pitting they require.
   2. The methods developed within PIT (electrochemical and coupon analysis) will be tuned in order to be able to deliver this type of data, and allow more accurate fatigue life calculations using existing fatigue models.

Target group

The PIT project primarily serves the Offshore Renewables (fixed foundations) and maritime sectors. However, cross-domain applications are expected in:

• Floating energy structures
• Energy islands
• Mooring systems
• Shipping and aquaculture
• Freshwater production
• Chemical industry
• Wastewater treatment 

Funding

• Project type: Cluster SBO, Blauwe Cluster
• Contract number VLAIO : Project nr HBC.2024.0692
• Funding level: 100% VLAIO funding

Project logo

Links

Internal links

The PIT project is a direct result of the work done in two projects MaxWind and Socorro, where a lack of understanding on pitting corrosion of carbon steel in a marine environment was detected after broad discussions with the industry. In particular, how to monitor the occurrence of pit corrosion is of interest to the industry.

External links

• Publication on FRIS portal
• Blue Cluster