The varieties of water throughout the world differ to the extent that predicting corrosion rates is very difficult for all coatings. Many parameters affect corrosion of metals in a water environment , including pH level, oxygen content, water temperature, water climate and tide conditions, to name a few. Despite the difficulty of predicting corrosion, it is important to note that galvanized coatings on steel used in submersed applications is still one of the best methods of corrosion protection. It is common for hot-dip galvanized steel to perform flawlessly in harsh water environments such as seawater for eight to 12 years.
The first step in deciding whether galvanized steel is the right coating for your application is to determine what type of water will be used. Water can be divided into a number of different types; pure water (e.g., distilled water or de-ionized water), natural fresh water, potable water (treated drinking water), or seawater. Hard water and soft water also cause corrosion to different degrees, as do hot and cold water.
View corrosion data for zinc/hot-dip galvanizing in specific water environments:
Pure water, also known as de-ionized or distilled water, is usually very corrosive to zinc coatings due to the presence of dissolved oxygen and carbon dioxide. Corrosion rates of steel increase with aeration of pure water; dissolved oxygen in pure water is five to ten times more aggressive than carbonic acid.
Natural Fresh Water
Fresh water environments have two major constituents for categorizing corrosion potential: hard and soft water. Carbonates and bicarbonates, present in some concentration in fresh water, tend to deposit protective films on the zinc surface, helping to stifle corrosion. Carbonates subdue the corrosion effects of anions, the most corrosive to zinc being chloride in excess of 50 mg/L. The softer the water, the lower it is in carbonate, which means a more pronounced chloride content. Conversely, the harder the water the greater it is in carbonate and the corrosiveness of the chlorides are minimized. Therefore, the general rule is that the corrosion rate of soft water is higher than hard water.
Seawater is high in salt content in the form of various chlorides. Typical surface seawater has a pH of 8 due to excess amounts of carbonates. The pH may fall to 7 in stagnant waters. The depth of the water also plays a part in the pH level. The pH decreases with depth. Corrosion of zinc is best controlled in the pH range of 5.5 to 12.
Seawater temperature can vary widely from 28 F (-2 C) at the poles to (95 F) 35 C near the equator. The higher the temperature the greater the dissolution of zinc in water. Tropical seawater (higher temperatures) yields higher corrosion rates, especially in polluted waters.
In the mid-1980s Congress passed the Clean Water Act, which includes the Drinking Water Standard. This standard requires that any material or coating that comes in contact with drinking water must be tested. The EPA contracted the National Sanitation Foundation (NSF) to write the test procedure, which after many drafts and public meetings, was finally published as NSF Standard 61: “Drinking Water Systems Components: Health Effects.” Therefore, only galvanizers that have submitted test coupons of their galvanized steel and have been approved by the NSF have the authority to galvanize steel for use with potable water. Despite the great lengths that a galvanizer must endure to gain this certification, hot-dip galvanized steel is a very suitable application for potable water.
Tidal Zones & Water Agitation
Tidal zones and fluid agitation are also important considerations in determining the corrosion protection delivered by galvanized steel. Often this motion of “washing” the carbonates off the zinc surface and not allowing them to form a protective film, along with zinc erosion, can be the cause for accelerated corrosion of zinc coatings.
Causes of Zinc Corrosion
Determining the corrosion rate of zinc in water can be a daunting task. Let’s list the basic elements that cause the corrosion of zinc:
- Soft water is a harsher environment to zinc than is hard water or even cold climate seawater.
- Temperate seawater deposits protective scales on zinc and is less corrosive than tropical sea water.
- High oxygen content and warm waters can increase the corrosion rates of zinc.
- Corrosion of zinc is lowest in the pH range from about 5.5 to 12.5. However, the corrosion of zinc in water is accelerated largely by the impurities present in the water and rarely is natural water pure. Even rainwater, which is distilled by nature, contains nitrogen, oxygen, carbon dioxide, as well as acquired airborne impurities such as dust or smoke.
- Anything that disturbs the formation of a protective film on the zinc surface will inhibit the hot-dip galvanized coating from delivering superior corrosion protection. In addition, other factors including time of exposure, temperature, and fluid agitation all influence the corrosion of zinc in water.