Attack by Seawater

The sensitivity of concrete towards the action of seawater is above all due to the presence of calcium hydroxide and hydrated calcium aluminates, which are both susceptible to reactions with sulfate ions from the seawater. Expansive effects decrease, therefore, as the percentage of CjA in the cement or the content of Ca(OH)2 in the concrete diminishes. The traditional approach to avoid these reac- 

Limiting C3A may, on the other hand, have an adverse effect on the chloride-penetration resistance of concrete. The use of cement with low CjA (such as ASTM type V) in many marine structures, in particular in The Middle East built in the 1970s and 1980s, has caused many of them to suffer from extensive reinforcement corrosion. Here it was wrongly assumed that a better resistance against sulfate attack would also mean a better resistance to all adverse effects of seawater [26]. 

The modern view is that the cements best adapted to seawater are blast furnace slag, fly ash and pozzolanic cemenf because a much lower amount of calcium hydroxide is present in the hydration products and the finer pore structure strongly reduces the transport rate of both sulfate and chloride ions. More detailed recommendations may vary on local experience. 

Concrete Structure, Properties, and Materials, Prentice Hall, 1993. 

Specification, Performance, Production and Conformity, European Committee for Standardization, 

---

The enrichment of ilmenite in beach sand in existing or fossil coastlines is important for 1102 production. The action of surf, currents, and/or wind results in concentration of the ilmenite and other heavy minerals such as rutile, zircon, monazite, and other silicates in the dunes or beaches. This concentration process frequently leads to layering of the minerals. Attack by seawater and air over geological periods of time leads to corrosion of the ilmenite. Iron is removed from the ilmenite lattice, resulting in emichment of the Ti02 in the remaining material. The lattice is stable... 

The most common coating inhibitors are zinc chromate and plumbous orthoplumbate (red lead), which passivate steel by providing chromate and plumbate ions, respectively, as well as the zinc and lead cathodic inhibitors. These inhibitors are not effective against attack by seawater or brines because the high chloride concentration prevents passivation of steel. 

Magnesium is rapidly attacked by seawater, many salt solutions, most mineral acids, methanol and ethanol, most wet gases, and halogenated organic compounds, when wet or hot. 

Vanadium is resistant to attack by hydrochloric or dilute sulfuric acid and to alkali solutions. It is also quite resistant to corrosion by seawater but is reactive toward nitric, hydrofluoric, or concentrated sulfuric acids. Galvanic corrosion tests mn in simulated seawater indicate that vanadium is anodic with respect to stainless steel and copper but cathodic to aluminum and magnesium. Vanadium exhibits corrosion resistance to Hquid metals, eg, bismuth and low oxygen sodium. 

General corrosion damage was the cause of failure of an A1 alloy welded pipe assembly in an aircraft bowser which was attacked by a deicing-fluid — water mixture at small weld defects . Selective attack has been reported in welded cupro-nickel subjected to estuarine and seawater environments . It was the consequence of the combination of alloy element segregation in the weld metal and the action of sulphate reducing bacteria (SRB). Sulphide-coated Cu-enriched areas were cathodic relative to the adjacent Ni-rich areas where, in the latter, the sulphides were being continuously removed by the turbulence. Sulphite ions seemed to act as a mild inhibitor. 

One of the most notable features of seawater is its high degree of saltiness. In previous chapters, we have discussed various sources of this salt, these being rivers, volcanic gases, and hydrothermal fluids. These elements have ended up in one of four places (1) as dissolved ions in seawater, (2) as sedimentary minerals, (3) as hydrothermal minerals, and (4) as volatiles that reside in the atmosphere. The minerals are recycled via geologic uplift and subduction. Upon return to Earth s surface, these minerals are chemically weathered via acid attack by the atmospheric volatiles remobilizing the salts for return to the ocean in river runoff. 

NS-300 Membrane. The NS-300 membrane evolved from an effort at North Star to form an interfacial poly(piperazine Isophthala-mide) membrane. Credali and coworkers had demonstrated chlorine-resistant poly(piperazineamide) membranes in the asymmetric form (20). The NS-lOO, NS-200, and PA-300 membranes were all readily attacked by low levels of chlorine in reverse osmosis feedwaters. In the pursuit of a chlorine-resistant, nonbiodegra-dable thin-fiim-composite membrane, our efforts to develop interfaclally formed piperazine isophthalamide and terephthalamide membranes were partially successful in that membranes were made with salt rejections as high as 98 percent in seawater tests. 

Iron-nickel alloys tend to be of lower corrosion resistance than iron-chromium alloys except towards attack by hot concentrated alkalis. Iron-chromium-nickel alloys are superior to either of the above and are resistant to alkaline and neutral aqueous solutions, atmospheric and seawater attack. Hot non-oxidising acids will cause corrosion, the rate depending on concentration and temperature. 

Properties Silvery solid or dark-gray, amorphous powder. D 4.6 (20C), mp 1675C, bp 3260C, sp heat 0.13 Btu/lb/F, thermal conductivity 105 Btu/ft2/F/ hour, as strong as steel but 45% lighter, Vickers hardness 80-100, excellent resistance to atmospheric and seawater corrosion and to corrosion by chlorine, chlorinated solvents, and sulfur compounds reactive when hot or molten. Insoluble in water, inert to nitric acid but attacked by concentrated sulfuric acid and hydrochloric acid. Unaffected by strong alkalies. 

Marine borers present a severe challenge. Some preservatives that are very effective against decay fungi and insects do not provide protection in seawater.Thus, despite severe reservations about the continued use of creosote and CCA these remain the only viable treatments currently available. Creosote is most commonly used, preventing attack by all marine borers except Limnoria tripunctata. 

Massive zirconium has excellent resistance to chemical attack by water, seawater, or steam... 

---