Sea water from underground sources

Salt water from underground sources frequently has a much higher salt content than sea water, quite often it is about 30%, i.e. just under the solubility limit. Here again, common salt is the major constituent. The pH value is usually comparatively low (down to about 4), i.e. the water is acidulous. Whereas the content of oxygen is very low or even nonexistent, the H₂S content may amount of a few hundred milligrams per litre.

Such acidulous salt solutions containing H₂S are very corrosive and call for special materials.

As a consequence of the high salt content and depending on the operating conditions, one must expect a certain degree of salt precipitation. In such cases, suitable countermeasures must be taken in respect of the design, operation and the selection of material.

Corrosion in sea water

The materials employed have not only to exhibit sufficiently high resistance to uniform corrosion, but also against local corrosion particularly pitting and crevice corrosion. Such corrosion phenomena are experienced especially with selfpassivating ferro alloys (stainless steels). So-called ‘standby’ pumps, which are only operated intermittently, run the risk of standstill corrosion; flooding with fresh water prior to a shut-down period or periodic start-up is considered advantageous.

The various sea water pump components should be made from materials of the same type in order to prevent galvanic corrosion. The potential difference between the individual materials is to be as low as possible. However, if unlike materials have to be employed for design reasons, the surfaces of the less noble metal in contact with the water should be large in comparison with those of the noble metal. Figure 5 provides information on the danger of galvanic corrosion when different type materials are combined.

High velocities can lead to erosion corrosion. The consequences become increasingly serious, the more aggressive the medium, and the higher its velocity. Whereas the flow rate affects the behaviour of stainless steels and nickel alloys to only a minor degree, the position is reversed where unalloyed ferrous materials and copper alloys are involved. Figure 6 provides qualitative information on the influence of flow rates. Due consideration must be given thereby as to whether the medium contains oxygen or H₂S. Large quantities of H₂S tend to exclude the presence of oxygen; in such cases, the medium is slightly acidulous, down to a pH of 4.

Material behaviour

Table 1 makes recommendations for pump materials or their combinations. Unless otherwise stated, the following information applies for sea water without any H₂S content.

Unalloyed steel and cast iron

Unalloyed steel is unsuitable for sea water, unless it is provided with a suitable protective coating. Cast iron is only to be used for low velocities (possible for casings); in this case normal cathodic protection of the other internals should be employed.

Austenitic Ni-castings

Ni-Resist 1 and 2 are only suitable for medium velocities (up to about 20 m/s).