The Effect of Temperature and Local pH on Calcareous Deposit Formation in Damaged Thermal Spray Aluminum (TSA) Coatings and Its Implication on Corrosion Mitigation of Offshore Steel Structures

This paper is based on experimental data and provides better understanding of the mechanism of calcareous deposit formation on cathodically polarized steel surfaces exposed to synthetic seawater at 30 °C and 60 °C. The study comprises measurement of the interfacial pH of thermally sprayed aluminum (TSA) coated steel samples with and without a holiday (exposing 20% of the surface area). Tests were conducted at the corrosion potential for up to 350 h. It was experimentally determined that the local pH adjacent to the steel surface in the holiday region reached a maximum of 10.19 and 9.54 at 30 °C and 60 °C, respectively, before stabilizing at about 8.8 and 7.9 at the two temperatures. The interfacial pH on the TSA coating at 30 °C was initially 7.74 dropping to 4.76 in 220 h, while at 60 °C it increased from pH 6.41 to the range pH 7.0–8.5. The interfacial pH governed the deposition of brucite and aragonite from seawater on the steel surface cathodically polarized by the TSA. This mechanism is likely to affect the performance of TSA-coated offshore steel structures, especially when damaged in service.

Related papers:

title: Corrosion science and technology
abstract: Overview of Corrosion and Protection Strategies Corrosion in Aqueous Media Thermal Oxidation Environmentally Sensitive Cracking Strategies for Corrosion Control Some Symbol Conventions and Equations Structures Participating in Corrosion Processes Origins and Characteristics of Structure The Structure of Water and Aqueous Solutions The Structures of Metal Oxides The Structures of Metals Thermodynamics and Kinetics of Corrosion Processes Thermodynamics of Aqueous Corrosion Kinetics of Aqueous Corrosion Thermodynamics and Kinetics of Dry Oxidation Mixed-Metal Systems Galvanic Stimulation Galvanic Protection The Intervention of Stress Environment-Sensitive Cracking Enhanced Corrosion in Flowing or Turbulent Aqueous Media Stress-Corrosion Cracking (SCC) Corrosion Fatigue Enhanced Corrosion in Flowing or Turbulent Aqueous Media Precautions against Stress-Induced Failures Protective Coatings Surface Preparation Electrodeposition Hot-Dip Coatings Conversion Coatings Paint Coatings for Metals Corrosion of Iron and Steels Iron and Steel Microstructures Rusting Oxidation of Iron and Steels Stainless Steels Phase Equilibria Commercial Stainless Steels Resistance to Aqueous Corrosion Resistance to Dry Oxidation Applications Applications of Cast Stainless Steels Corrosion Resistance of Aluminum and Aluminum Alloys Physical Metallurgy of Some Standard Alloys Corrosion Resistance Corrosion Resistance of Copper and Copper Alloys Chemical Properties and Corrosion Behavior of Pure Copper Constitutions and Corrosion Behavior of Copper Alloys Corrosion Resistance of Nickel and Its Alloys Chemical Properties and Corrosion Behavior of Pure Nickel Constitutions and Corrosion Behavior of Nickel Alloys Corrosion Resistance of Titanium and Its Alloys Chemical Characteristics and Corrosion Behavior of Pure Titanium Constitutions and Corrosion Behavior of Titanium Alloys Applications Cathodic Protection Principles Buried Pipelines and Distribution Systems Cathodic Protection in Open Waters Side Reactions and Overprotection Measuring Instruments Corrosion and Corrosion Control in Aviation Airframes Gas Turbine Engines Corrosion Control in Automobile Manufacture Overview Corrosion Protection for Automobile Bodies Corrosion Protection for Engines Bright Trim Corrosion Control in Food Processing and Distribution General Considerations The Application of Tinplate for Food and Beverage Cans Dairy Industries Brewing Control of Corrosion in Building Construction  Cladding Metal Roofs, Siding, and Flashing Plumbing and Central Heating Installations Corrosion of Metals in Timber Application of Stainless Steels in Leisure Pool Buildings Corrosion Control in Marine Environments Natures of Marine Environments Ships Offshore Platforms Corrosion Control in Steam-Raising Plant for Power Generation Fossil Fuel-Fired Boilers Recirculating Pressurized Water Reactor (PWR) Steam Generators Principles of Corrosion Testing Accelerated Tests Exposure Tests Pilot Tests Stress-Enhanced Corrosion Tests Tests for Resistance to Thermal Oxidation Index

title: Understanding material interactions in marine environments to promote extended structural life
abstract: Corrosion in marine environments is affected by how and to what extent seawater interacts with materials, by the choice of material, and by design of components, systems, and structures. Seawater is a complex, naturally buffered fluid that covers much of the Earth’s surface. The chloride content and the dissolved oxygen content in seawater promote competing processes of passivation and the breakdown of passivity. Seawater is a living medium where microbiological organisms thrive and attach to virtually all materials leading to biofilm formation and micro- and macrofouling. Microbiological activity may accelerate corrosion processes or act as a passive barrier. Biofilms have been shown to ennoble materials in seawater after an induction period. The behavior of materials, particularly metals and alloys, in marine environments generally depend on the chemical composition, the stability of the oxide film, metallurgical and processing parameters, and the effectiveness of any applied protection and mitigation measures. Materials selection and appropriate design can avoid, delay, or minimize the occurrence of several forms of corrosion. The proper choices require an complete understanding of materials, design factors, the type of marine environment, and the relevant corrosion control methodologies in order to promote increasing service life to marine structures. This paper will discuss some of the parameters that affect materials performance and some effective measures by which alloys can be protected in selected marine environments.

title: Calcareous scales formed by cathodic protection—an assessment of characteristics and kinetics
abstract: An electrochemical technique using the assessment of the rate of oxygen reduction at a rotating disk electrode has been used in conjunction with surface analysis by X-ray photoelectron spectroscopy to study the first layer formation of calcareous deposits under cathodic protection in various solutions. The study has shown that the electrochemical technique is an effective means of monitoring scale formation and that the existing models which predict scale formation by a basal layer of Mg(OH) 2 followed by CaCO 3 are over-simplified. The advantages of using an integrated approach of electrochemical analysis and a surface sensitive analysis technique in characterising the initial scale as a function of the solution composition are presented and discussed and revised models describing the temporal development of calcareous layers in saline solutions are suggested.

title: Calculation of Seawater pH at Polarized Metal Surfaces in the Presence of Surface Films
abstract: A model was developed for calculating the pH at a cathodically polarized metal surface. The model accounts for the buffering capacity and ionic strength of seawater, and it is valid in the...

title: Dissolved Oxygen and pH Microelectrode Measurements at Water-Immersed Metal Surfaces
abstract: Dissolved oxygen (DO) and pH were measured at metal/artificial seawater interfaces using microelectrodes in biotic and abiotic systems. Measurements in a closed system proved that presence of electrochemical and/or biological reaction products substantially influence the conditions at the metal surface. For long-term studies, only open (e.g., continuous flow) reactors should be used. An open channel flow reactor suitable both for microbiological and electrochemical measurements has been constructed and successfully tested.

title: Perry's Chemical Engineers' Handbook
abstract: Conversion Factors.Physical and Chemical Data.Heat and Mass Transfer.Transport and Storage of Fluids.Heat Transfer Operations and Equipment.Drying, Humidification and Evaporative Cooling.Distillation.Gas Absorption and Other Gas-Liquid Operations and Equipment.Solid-State Operations and Equipment.Size Reduction and Size Enlargement.Handling of Bulk Solids and Packaging of Solids and Liquids.Alternative Separation Processes.Safety and Handling of Hazardous Materials.Energy Sources, Conversion and Utilization.Process Machinery Drives.Analysis of Plant Performance.

title: Materials Crystal Chemistry
abstract: Introduction - crystal chemistry in materials science fundamentals of atoms, molecules and solids fundamentals of crystallography crystal structures structure change of crystals.

title: A Study of Calcareous Deposits on Cathodically Protected Mild Steel in Artificial Seawater
abstract: Calcareous deposits were formed on steel under conditions of cathodic protection in artificial seawater at applied constant current densities ranging from 50 to 400 mA·m−2. The calcareous layers were characterized using a Field Emission Gun Scanning Electron Microscope (FEG SEM) in conjunction with Energy Dispersive X-Ray Analysis (EDX), and Electrochemical Impedance Spectroscopy (EIS). At cathodic current densities of 50–100 mA·m−2 where corrosion was still occurring, a clear correlation existed between the iron containing corrosion product and the overlying magnesium hydroxide layer. This revealed that the mapping of magnesium rich areas on a steel surface can be used in the identification of local corrosion sites. At current densities of 150–200 mA·m−2, a layered deposit was shown to occur consisting of an inner magnesium-containing layer and an outer calcium-containing layer. At current densities of 300–400 mA·m−2, intense hydrogen bubbling through macroscopic pores in the deposits gave rise to cracking of the deposited film. Under such conditions deposits do not have a well-defined double layer structure. There is also preferential formation of magnesium-rich compounds near the steel surface at the early stages of polarisation and within the developing pores and cracks of calcareous deposits later on. Based on SEM/EDX investigation of calcareous depositions the impedance model was proposed and used to monitor in situ variations in steel corrosion resistance, and to calculate the thickness of formed deposits using the length of oxygen diffusion paths.

title: General guidelines for corrosion testing of materials for marine applications: Literature review on sea water as test environment
abstract: AbstractThis literature review is intended to give some general guidelines on corrosion testing of metals and alloys for marine applications, in particular with regard to the choice of the test environment. The general chemical properties of sea water are discussed and compared with those of coastal and/or polluted sea water, stored and recirculated sea water, and synthetic solutions which are sometimes used to simulate sea water. After discussing the factors that can affect the corrosion mechanism, the results of test methods are reviewed, highlighting the differences obtained in the various test environments. A short review of the main alloys used in marine applications is given, followed by a discussion of the factors that can lead to differences between results obtained when testing in natural sea water and in simulated test solutions.

title: Corrosion of Aluminum and Aluminum Alloys
abstract: This volume presents comprehensive coverage of the corrosion behavior of aluminum and aluminum alloys, with emphasis on practical information about how to select and process these materials in order to prevent corrosion attack. Described are the characteristics of these materials and the influences of composition, mechanical working, heat treatment, joining methods, microstructure, and environmental variables on their corrosion.

title: Stabilization of corroded aluminium
abstract: AbstractA simple washing procedure has been developed for stabilization of corroding aluminium-copper alloys. The use of an ammonia-ammonium sulphate buffer at pH 9·6 in aerated deionized water effectively removes cemented copper metal and copper corrosion products from the surface of the object whilst also removing aggressive chlonde ions. This treatment has been used to stabilize a Duralumin sea-plane float which was actively corroding after 46 years' exposure to a marine environment.

title: Calcareous Deposits on Metal Surfaces in Seawater—A Critical Review
abstract: While the importance of calcareous deposits to the effectiveness and efficiency of marine cathodic protection is generally recognized, there has been relatively little research directed to...

title: Influence of Cathodic Protection Parameters on Calcareous Deposit Formation
abstract: Formation of calcareous deposits has the beneficial effect of reducing the current requirement of cathodic protection. This study was focused upon the current-reducing capability of the deposits and its relationship with influential parameters of cathodic protection. Four experimental parameters were selected and investigated over a range that reflects typical seawater conditions. The parameters were applied cathodic potential, flow velocity (actually, rotation speed of electrode), pH, and temperature. Deposits were formed in artificial seawater. Influence of each of the four parameters on cathodic current was obtained by using the factorial design technique. Within the range studied, it was found that pH has the largest influence and temperature the least. This was explained based upon the influence of the parameters on deposit formation and electrochemical reactions.

title: The chemistry of gold extraction
abstract: Historical developments process mineralogy process selection metallurgical development principles of gold hydrometallurgy oxidative pretreatment leaching solution purification and concentration recovery surface chemical methods refining effluent treatment industrial applications.

title: Role of sulphate ions on the formation of calcareous deposits on steel in artificial seawater; the formation of Green Rust compounds during cathodic protection
abstract: Cathodic protection of steel in seawater could be optimized by taking into account the calcareous deposits forming on the steel surface. The aim of this work was to study the influence of sulphate ions on the kinetics and mechanisms of formation of these deposits. The experiments were performed at 20 °C, with an applied potential of −1.0 V/saturated calomel electrode (SCE), in artificial seawater-like solutions with various SO 4 2− concentrations. The deposition was monitored by chronoamperometry and electrochemical impedance spectroscopy (EIS). Micro-Raman analyses were performed in situ in a specific electrochemical cell to identify the solids forming on the steel surface. It could be demonstrated that sulphate ions had an important effect on the formation of both Ca(II)- and Mg(II)-containing phases. In the solution enriched with sulphate ions, the deposition of CaCO 3 was almost totally inhibited. Experiments performed in Ca 2+ -free solutions demonstrated that the Mg-based deposit was, in contrast, favoured by the increase of the sulphate concentration. In situ Raman spectra of the solid forming at the early stages of the cathodic protection proved to be characteristic of Green Rusts (GRs). This compound was favoured by the presence of Ca 2+ and/or Mg 2+ cations, and is more likely a GR-like M(II)-Fe(III) hydroxysulphate, with M = Fe, Mg and Ca.