Corrosion is responsible for the failure of many systems and structures. Corrosion involves the reaction of a metallic material with its environment and is a natural process in the sense that the metal is attempting to revert to the chemically combined state in which it is almost invariably found in the earth’s crust.
To the engineer, corrosion may be regarded as resulting in a variety of changes in the geometry of structures or components that invariably lead, eventually, to a loss of engineering function e.g. general wastage leading to decrease in section, pitting leading to perforation, cracking leading to fracture.
On 16th September, 2008 a canopy collapsed at Comcraft house building in Nairobi. Investigations revealed the effects of weather on the Rectangular Hollow Section beam which was supporting the concrete canopy. This raised the question of the effects of corrosion and whether proper measures for prevention of corrosion were used. Though the quality of concrete was good the RHS beam had severe corrosion. On 10th May, 2018 a staircase collapsed at Mombasa old town due to corrosion effect.
The Government is spending a lot of money in rehabilitation of Jetties e;g Mokowe Jetty in Lamu, New terminal Jetty, Customs Jetty and Shimoni Fisheries Jetty. Investigations revealed that the reinforcement bars were corroded leading to failure of some beams and slabs.
The Customs Jetty collapsed in 2007 due to the effects of rust. The area MP and some people were injured.
Several other structures have collapsed because of corrosion. Corrosion of marine structures is a major challenge.
Statement of the Problem
Corrosion is degradation of materials’ properties due to interactions with their environments, and corrosion of most metals is inevitable. While primarily associated with metallic materials, all material types are susceptible to degradation. Degradation of marine structures has been a concern in aging ships. Like death and taxes, corrosion is something that should be avoided; but ultimately it is something that must be dealt with.
Corrosion of marine metal structures is a major cause of damage to these structures. The spilling of various facade cladding materials caused by the corrosion products frequently presents a major risk to public safety.
It has can be noted that a number of steel structures have been affected by corrosion in our country. The economy of the country is affected since the developers lose millions of shillings due corrosion issues. This inspection was done to investigate the challenges of corrosion on marine structures, evaluate their causes and the extent this has affected the marine structures in Kenya.
Objectives of the Inspection
Objectives of the Audit of the structures were:
- To evaluate factors which aggravate marine corrosion;
- To explore the various forms of marine corrosion;
- To examine the effect of the marine corrosion on structures;
- To find out how safe are the various marine structures affected by corrosion;
- To establish the extent of chemical corrosion on marine structures;
To evaluate the various measures taken to prevent marine corrosion.
Nowadays a great amount of damages that happens due to corrosion is observed.
In the United States resources at an order of millions of dollars are spent over the last years in inspections and rehabilitation programs, mainly in the marine structures (Stewart and Rosowsky, 1998). So, many studies are conducted with the purpose of identifying the degradation forms and providing information to minimize this kind of corrosion occurrence in new marine structures.
Marine corrosion is an electrochemical reaction that happens when electrons flow between metals that are connected or grounded through water.
As it happens, this electrical action causes one of the two metals to literally be eaten away. The process is greatly accelerated in salt, brackish waters, or in waters with a high mineral content.
Some factors that affect corrosion rates in marine atmospheres are humidity, wind, temperature, airborne contaminants, location and biological organisms. Alloy selection, metallic coatings, organic coatings, and cathodic protection are commonly used methods for providing proper corrosion protection to various components.
Marine corrosive Elements
The process of corrosion requires four essential elements namely: an Anode, a Cathode, an Electrolyte, and a Metallic path. Corrosion is a natural process that cannot be prevented, but intervention with the correct measures can control it.
Seawater readily supports corrosion cells as seawater is nearly 100% ionized as well as being considered very conductive.
Forms of corrosion
Uniform or General Corrosion
General corrosion, also referred to as uniform corrosion, is one of the most common types of corrosion. It damages the entire surface of the material at about the same rate, causing the metal to thin.
Pitting corrosion causes damage by randomly attacking a limited section of the metal’s surface, leaving behind holes that are larger in depth than width. The “pit” that forms functions as the anode and the metal that is left undamaged functions as the cathode.
Stress Corrosion Cracking
Stress corrosion cracking (SCC) is a complex form of corrosion that occurs when brittle, dry cracks develop from the combined effects of a tensile stress and a corrosive environment. Stress corrosion affects only a section of material; leaving most of the material unaffected.
Inter granular Corrosion
The microstructure of metals and alloys consists of a granular composition. Inter granular corrosion, also called inter crystalline corrosion, occurs on or adjacent to the grain boundaries of a metal. Some causes of inter granular corrosion are welding, stress annealing, improper heat treating or overheating in service.
Crevice corrosion, also called concentration cell corrosion, forms when a liquid corrosive is trapped in narrow gaps of space between metals, or between nonmetals and metals. Aggressive ions like chlorides must be present in the electrolyte. The design of certain materials, such as gaskets, fasteners, surface deposits, washers, threads, clamps, and any close-fitting surfaces often initiates crevice corrosion. Crevice corrosion can be extremely aggressive; its detection and monitoring is a major challenge.
Factors which aggravate marine corrosion
Pitting and Cavitation
The highest water velocities, at the tips of propellers or in pumps can result in bubbles of entrained air imploding with sufficient energy to remove metal or break up composites.
High levels of stress in service, or residual stress from manufacturing may result in selective corrosion of more highly stressed regions of an otherwise corrosion resistant structure.
Methods of Protection From Corrosion
- Protection by painting
- Cathodic protection
- Galvanic corrosion
- Using corrosion resistant alloys
Prevention of marine corrosion
There are five main methods for controlling the tendency of metals to corrode in sea water:
- By isolation of the corroding metal from the sea water by painting, or other coating
- Changing the potential of the metal to a point where corrosion ceases – by impressed voltage or coupling to a sacrificial anode.
- By making the metal passive, using corrosion inhibitors.
- Changing the pH of the local environment by chemical dosing.
- And finally by making a change to a more corrosion resistant material
Methodology of carrying the study.
The study was carried out through inspections, interviews, taking underwater photographs and analysis of existing documents for ongoing works.
Corrosion can reduce the load-carrying capacity of a component either by reducing its size or by pitting. Pitting not only reduces the effective cross section in the pitted region but also introduce stress raisers, which initiate cracks. The three interrelated factors that drive the selection of materials for corrosion control are the corrosivity of the environment, the corrosion resistance of the material and the acceptable rate of attack (Neff, 2000).
The marine growth was noted at splash zone as massive, offering natural corrosion protection. The substructure had marine growth which offered protection. But on the substructure, Chloride attack on the piles leading to massive corrosion were noted.
Spalling of concrete on slabs, beams and columns on buildings audited.
Prevention of corrosion by painting was carried out on steel chains carrying rubber fenders at Mokowe, New Terminal and Customs Jetties at Lamu. The same was done for Shimoni Fisheries Jetty.
Ensuring proper cover to concrete elements such as beams, columns and piles.
It was observed that the concrete piles with a cover of 60mm and above resisted the corrosion more than any which was less. Proper marine designs are necessary.
Use of appropriate materials for marine construction. Timber fenders were not affected and proofed a better material for fenders.
Proper marine designs are recommended for marine structures. Concrete piles resisted corrosion more than steel piles. Appropriate corrosion resistant materials need to be used for piling works.
Concreting of the pre-cast concrete pile with 75mm cover
“Like death and taxes, corrosion is something that should be avoided; but ultimately it is something that must be dealt with.”