Keith E. Lucas

Microstructural inhomogeneities and sea water corrosion in laser-deposited Ti–6Al–4V alloy matrix/carbide particulate composite surfaces

Metal matrix/carbide particulate composite surface layers were produced on Ti–6Al–4V alloy samples by injecting metal carbide particles into laser-melted surfaces followed by rapid solidification. Hard, wear-resistant surfaces were produced on a strong alloy which normally has poor wear resistance. The corrosion behaviour of the composite surface was evaluated after a months exposure to flowing sea water. A variety of solidification products was found in the laser-deposited surface layers, but corrosion was observed only in the carbide particulate phase in the WC-injected sample. No corrosion was observed in the TiC-injected sample nor in the Ti–6Al–4V base alloy. Corrosion in the WC-injected sample was related to the formation of a narrow interphase zone surrounding the particulate phase and a thin reaction zone on the surface of the particulate phase during solidification. The titanium-rich interphase zone formed a galvanic couple with the WC particulate. Crevice-type corrosion initiated at the interface between the two phases and proceeded into the particulate phase assisted by the reaction zone. Electrochemical test results revealed a high corrosion rate for the WC-injected sample and almost none for both the TiC-injected sample and the Ti–6A1–4V base alloy, confirming the microstructural observations.

Crevice corrosion of Alloy 625 in natural seawater

An experimental study was conducted to determine the influence of temperature on crevice corrosion initiation for Alloy 625 (UNS N06625) in natural seawater. These tests showed that that there was a critical potential–temperature–time relationship needed to initiate crevice corrosion. The potential necessary to cause crevice corrosion on Alloy * A. M. Grolleau contributed to this work while a visiting scientist at the Naval Research Laboratory in Key West, FL as part of the U. S./ French Defense Exchange Program 625 decreased (became less noble) when the temperature was increased from ambient to 40°C. The crevice initiation potential decreased from 300mV for ambient temperature seawater to around 100mV for 40°C seawater. Crevice initiation potentials were essentially unchanged between 40°C and 65°C, while the time required to initiate crevice corrosion decreased as temperature increased. In a second aspect of this work, natural seawater exposure studies were conducted to determine if there is a mechanistic connection between ennoblement (the gradual elevation of corrosion potential that occurs during long-term continuous immersion in natural seawater) and crevice corrosion initiation. It was found that ennoblement produced corrosion potentials for an extended period of time that exceed the crevice corrosion initiation potential in ambient temperature natural seawater. Temperature transients from ambient to elevated temperature created temporary conditions where the corrosion potential was substantially higher than the crevice initiation potential for short periods of time – but only if ennoblement had previously occurred at ambient temperatures.

Crevice Corrosion Propagation on Alloy 625 and Alloy C276 in Natural Seawater

Abstract Chemical composition of the aqueous solution within crevices on two different Ni-Cr-Mo-Fe alloys immersed in natural seawater was determined using a semiquantitative thin-layer chromatogra...

Environmental fate of sacrificial zinc anodes and influence of a biofilm

Laboratory experiments were performed to determine the environmental fate of zinc compounds formed during sacrificial dissolution of zinc anodes in seawater and to further determine the role of biofilms in mobilizing/immobilizing zinc corrosion products. Experimental conditions simulated cathodic protection and overprotection (40x normal) of ship ballast tanks with an anode to cathode ratio of 0.091. Anodic dissolution and percentage of soluble zinc increased with the level of protection. For normal cathodic protection in abiotic seawater, approximately 3% of the sacrificed zinc was dissolved in solution. Additionally, weight loss and percent soluble products increased dramatically in the presence of an agar film, but were unchanged due to the presence of natural marine biofilms. Under overprotection conditions, weight loss did not vary in the presence of surface films. Solid zinc corrosion products were precipitated, suspended in solution, and attached to electrode surfaces. Experimental data were compared with zinc concentrations measured in ballast tanks for two United States Navy surface ships.

Boundary Element Evaluation of ICCP Systems Under Simulated Service Conditions

The cost of repairing damage to marine structures and ships attributed to corrosion have lead to the development of technologies which try to limit the corrosion process. Use of cathodic protection systems take advantage of the electrochemical nature of the corrosion processes and attempt to limit the degree of corrosion which occurs. External sources of electrical current are used in impressed current cathodic protection (ICCP) systems. Current is supplied through source anodes and voltage levels are monitored through reference cells. ICCP systems are currently in use on a wide variety of structures.

Design of Impressed Current Cathodic Protection (ICCP) Systems for U.S. Navy Hulls

The goal of impressed current cathodic protection (ICCP) design for ship hulls, under the Navy Ships Technical Manual (NSTM, Chapter 633), is to provide a uniform potential distribution at {minus}0.85 V, {+-}0.05 V, versus a silver/silver chloride (Ag/AgCl) reference cell, over the wetted hull surface during all operational aspects of an active ship. To accomplish this, the physical scale modeling (PSM) technique, combined with a rigid design protocol, has been used extensively by the US Navy to provide optimal and retrofit upgrade designs of ICCP systems for hulls. The ICCP design guidance, provided by the protocol, defines the hull properties, hull damage and general power supply requirements. PSM is utilized to determine optimal placement of ICCP components (anodes and reference cells) and to evaluate performance for up to a 15% wetted hull coatings loss under static (pierside) and dynamic (underway) conditions. Data are provided which illustrate the use of the design protocol criteria, along with the integrated PSM technique, to determine ICCP system design and evaluate performance.

Computational design of ICCP systems: Lessons learned and future directions

Computational modeling using boundary element techniques has been proposed for design and evaluation of shipboard impressed current cathodic protection (ICCP) systems. LaPlaces equation, the governing differential equation for electrochemical corrosion, is well suited for solution by the boundary element method. There has been much work performed in this field during the past two decades. Computational modeling efforts designed to validate boundary element procedures are reviewed. U. S. Navy ship systems discussed are CG-59, CG-66 and CVN-68. Computational analysis accuracy is determined by comparison with physical scale modeling experimental results. Lessons learned from the analyses described are summarized. Advantages and disadvantages of boundary element modeling are discussed.

6 – 70/30 copper–nickel seawater piping systems – use of descaling agents and their effects on corrosion properties

Publisher Summary This chapter focuses on the 70/30 copper–nickel seawater piping systems and highlights the use of descaling agents and their effects on corrosion properties. Use of copper–nickel alloys in seawater-cooled heat exchangers is common due to the inherent properties of these alloys—namely, a relatively low corrosion rate, antifouling properties, and erosion–corrosion resistance relative to other alloys. The main causes of failure of copper–nickel alloy components in seawater are usually attributed to either erosion–corrosion at extreme velocities or the pitting phenomena. It is well established that these failures are generally due to detrimental seawater conditions, such as the presence of sulphides or poor designs resulting in extreme flow velocities. Periodic cleaning is necessary on seawater cooling systems due to the gradual formation of scaling and macro-fouling during operation. Several chemicals and procedures can be used to remove these deposits and growths, the most common products being hydrochloric, citric or sulfamic acids. Hydrolancing and mechanical cleaning are in use by the fleet, but these have major drawbacks in terms of costs and labor hours. In addition, most of these methods raise some safety and environmental issues. Three commercial descaling solvents, two hydrochloric acid-based and one phosphoric acid-based, have been identified that claim to be safe, environmentally friendly, and inexpensive and exhibit no detrimental effects on the materials.

Generation and fusion of multiple image representations in automatic corrosion detection algorithms used with shipboard tank video inspection systems

Coatings damage in shipboard tanks is presently assessed using Certified Coatings Inspectors. Prior to a coatings inspector entering a tank, the tank must be emptied and certified gas free. These requirements combined with the limited number of certified coatings inspectors available at shipyards and Naval Bases significantly increases the cost and the logistical requirements associated with performing shipboard tank inspections. There is additionally significant variation in damage assessments made by different inspectors. To overcome these difficulties, the Naval Research Laboratory has developed two video inspection systems that obviate requirements for both certifying tanks gas free and for emptying the tank prior to performing an inspection. These systems also obviate requirements for inspector presence during tank inspections. The Naval Research Laboratory has also developed an automatic corrosion detection algorithm. The corrosion detection algorithm currently employs two independent algorithms that individually assess the tank coatings damage. The independent damage assessments are than fused to attain a single coatings damage value. In testing performed to date, it has been shown that the corrosion detection algorithm significantly reduces the effect of inspector-to-inspector variability and provides an accurate assessment of tank coatings damage. This in turn makes it significantly easier to prioritize ship maintenance.

FASTENERS IN MARINE SERVICE

In this study, the galvanic effect of non-ferrous fasteners materials with similar base plate were investigated. Five candidate nickel based fastener materials were immersed in natural flowing seawater at ambient temperature and pressure. These materials were electrically attached to Alloy 625 (NO6625) with greater than ten times the surface area. Corrosion currents and open circuit potential were measured for twenty-eight to one hundred and five days. Corrosion rates were calculated from nominal current densities and are presented. Results indicated that nickel base fasteners alloys may suffer from galvanic corrosion even in cases where open circuit potentials were similar to the structural material.