Corrosion

Che Practical Planter

Mints On Combating Costly

There are arguments against the need for corrosion protec tion, and sometimes they seem to be valid. But in most cases it pays—in reduced component failure, better per formance, longer life, and certainly the better mental atti tude of any operator who works with properly and fully maintained equipment—whether it's a shed or machine.

A fact that any metal article ncture must eventually corrode, corrosion is a process whereby is transformed into a solution compound state as a result of :al or electro-chemical reaction ts environment, distinction, in our case, be chemical and electro-chemical ion is not of major import and only indicates the pos r of reaction in the presence of (electrochemical, the chemical m is accompanied by a flow of c current) or without water ical, at high temperature, etc.), rosion as we see it, always is in the presence of water, so ■ elaboration we will deal only his type of phenomena, life of a metallic article, apart ivear, depends on its protection, le factors governing the life of rotective coating are:— ture of the surrounds to which s structure is exposed, sign of the structure. ■face preparation of the metal. )tective coating on the metal.

Surroundings

most critical atmospheric con affecting the life of steel is the of pollution, although the il, temperature, and angle and of exposure to the prevailing and rain are important, i and steel do not rust appreci n the air, whether polluted or >rovided the relative humidity lot exceed about 70 per cent, air is not polluted, this value e exceeded, but there is a rapid ration if there is over 70 per humidity and sulphur dioxide r other contaminants present.

The steelwork is sometimes exposed inside a structure such as a shed.

The protection needed is much less than that required for external ex posure but care must be taken to avoid condensation in roof spaces and in cavity walls, particularly if there is also a chance of contamin ation by flue gases and smoke from the burning of various types of fuel.

Care should also be taken to pre vent steelwork being in contact with materials that liquify, or become or remain damp for any length of time.

It should be remembered that certain types of timber are liable to produce acids which may accelerate corrosion under damp conditions.

Design

An important contribution to effective protection can be made by the avoidance of any detail that makes it possible for any moisture or dirt to accumulate, or that prevents the effective application of protective measures. Once steel has started to corrode, the rust will retain moisture and salts, and the corroding processes may be accelerated.

Serious corrosion in structures can also be caused by inadequate initial protection or neglect in maintenance and, therefore, back-to-back angles, beams, etc., should be avoided be cause of the difficulty of preparing the surfaces in situ and protecting them initially. Later the task of main taining them becomes almost im possible.

For the same reason features such as rainwater pipes and gutters should be arranged so that as much space as practicable is left between them and the walls. Ease of fabrication, erec tion and finishing should also be con-

By Dr. L. Borsody,

technical

director, Semper-Seal Chemical Pty.

Ltd., Sydney.

sidered. The following are import ant: • Whenever possible, sharp features where moisture, liquids and solids can accumulate should be avoided, and all corners and contours should be rounded. • Construction of angles, channels and beams should be arranged so as not to leave catchment areas for liquids. If this is not possible the appropriate size and number of drain age holes should be provided. These should not only be kept clear from blockage but should also be carefully sited and attention paid to the disposal of the drainage. • Metal coatings such as those of zinc or of aluminium may be applied by one of the appropriate pro cesses. • The various methods of joining should be considered. For ease of protection butt-welded joints are pre ferable to those that are lap-welded.

If, however, it is essential to use the latter, all exposed edges should be treated in such a way as to prevent the access and retention of liquids and dirt in the crevices. • Any storage containers and tanks should be supported on legs so that free circulation of air is possible and condensation is pre vented. • Evaporation of condensed moisture is often retarded on sheltered horizontal surfaces such as those under eaves of buildings. Such features should be provided with “breathing” holes or given additional protection such as a coat of water resistant finishing paint. • Contact between dissimilar metals should be avoided because of the possibility of the production of corrosion cells.

This is particularly true when metals have very different electrode potentials, such as copper, and iron or zinc. In the atmosphere, this bi metallic corrosion can be controlled provided that dissimilar metals are insulated; “rubbing” surfaces are made water-tight, and all surfaces are

sparing surfaces for a

ted or given a complete coating the same metal. is not sufficient to protect only less noble metals because there be severe attack of the baser il where there are any defects in ;oating.

Surface preparation

ne of the most important factors ting the life of a steel structure ic degree of surface preparation, stigation has shown that the dura of protection given by any paint >cheme when applied over a well ared surface is many times that ined over one that has been ly prepared or not cleaned at s it comes from the rolling mills, :tural steel carries a thin layer ron oxide, known as millscale, h is produced during the rolling ess by the action of the air on red-hot steel. A coat of paint be applied at any stage between rolling and the completion of the tion of a structure. . the majority of instances the ice condition of the steel will not be the same as when it left the ng mill, but it will have been ex d to the weather during the Dd before painting. Some mill ; will have been shed and any ily exposed steel surface will s rusted. If the paint is applied friable millscale the latter will itually become undermined by jture penetrating at weak spots will eventually burst away from steel, carrying the paint film with ust beneath the paint film need be quite so dangerous and gener results in blistering of the film; damage being caused, most >ably, by the expansion of the when it passes from a lower to igher degree of oxidation. How , rapid failure may occur if the is contaminated with certain utants. □ccessful protection can be lined only when the priming coat ipplied to a surface that is en y free from both millscale and he most effective methods for ieving this are mechanical and nical surface preparations. lechanical methods include sand, ; or grit blasting (wet or dry), le cleaning, grinding, wire brush and chipping. Of these vacuum blasting is becoming more widely

used because of its comparative clean liness, while flame cleaning and chip ping with air operated hammers are quite effective. Wire brushing and hand hammer chipping is extremely slow and often labourious.

In many cases, wire brushing and hand hammer chipping are the only practical means of removal, par ticularly in awkward places, but the removal of rust from deep pits is very necessary in the initial treat ment.

The essential requirement in the use of any equipment is to achieve a thoroughly cleaned, grease-free, dry surface. New steelwork must be cleaned of millscale and rust, and old steelwork cleaned of paint and mst scale.

Chemical methods

Pickling of steelwork is usually carried out by immersion, using a bath, at elevated temperature, of diluted sulphuric, hydrochloric or phosphoric acid. The first two are cheaper and have a more drastic action, but at the same time have a more corrosive effect on the metal; they strongly etch the steel and pro duce toxic fumes. For these and other reasons a phosphoric acid bath has become more acceptable where baths are practical.

The essential requirements for an acid pickling bath are:— • The incorporation of suitable inhibitors to prevent hydrogen em brittlement and reduce attack on the basic metal. • A low surface tension of the solution. • Effective concentration of acid to ensure penetration into layers of scale and rust. • Carefully controlled acid con centration of the bath, sludge forma tion and operating temperature.

One particular such agent known as “Ferrophos” is well known in the

field, because it can be applied to small as well as large operations. It is used for removal of rust or mill

scale from steelwork or cast iron, in cluding bolts and nuts.

Due to the phosphatic coating which is deposited on material passed through a bath of this nature the material can be placed under cover without further treatment for quite lengthy periods without development of rust. The general procedure is for the bath to be heated to about 140 deg. Fahr.—the work immersed for 10 to 20 minutes and then re moved and allowed to dry. Painting can be carried out after 24 hours, if necessary.

This is a treatment which descales, derusts and phosphates in one pro cess—it can be applied to general steelwork, preservation of stored parts and reclamation of old material.

In certain cases, equipment has rusted and must be cleaned in posi tion. This can be done by the use of Paste De-scaler. This de-scaler is quite powerful in its ability to re move solid rust and millscale, from new i in. flat, vertical or curved sur faces, for example. The period of reaction required for this paste will vary with the type and thickness of scale or rust, and may take anything from a few minutes to overnight. The essential point is that a paste is avail able to chemically remove scale and rust, eliminating hard work and causing considerable cost saving.

Conversion of rust

There are occasions when rusty equipment and structures have to be protected against further deteriora tion, but, economy and simplicity of operation are the foremost considera tion. This applies particularly in in stances where the metal to be treated is so badly corroded that it does not allow any further weakening through mechanical and chemical cleaning prior to application of the protective coating.

It is also often found that rust cleaning operations are not carried out properly due to inaccessability or lack of skilled supervision.

In all these cases it is far better to utilise the existing rusty surface by converting it into a useful and inhibitive phosphate coating. This can be achieved with specially formu lated rust converters or rust neutral isers, such as Ferropro Rust Con verter.

According to our findings, such a product should satisfy the following conditions:— • To wet thoroughly and infiltrate the rust particles and form a con tinuous bond throughout. • Sufficient penetration to replace

erosion adds to maintenance costs

moisture and vapour which may » been retained in the pores of metal. i To have a carefully balanced

nical proportion of phosphates

other chemicals in order to in the desirable conversion and the necessary corrosion inhibitive

sphate coating.

i Considering that such a pro ; is mainly used in the open, it ild incorporate a waterproof seal order to protect the phosphate ing formed against the elements I such time as painting can be * i the case of Ferropro Rust Con er, this seal is obtained during processing by forming a synthetic i which becomes fully cured after application. To obtain good re ; it is essential to first remove flaky and powdery rust, and then ; the surface with fresh water, he case of obvious contamination, addition of a small amount of rgent, about 1 oz to 5 gals, water, assist appreciably. Ferropro t Converter is then applied to the rust which has a strong chemical 3 to the basic metal, is essential to complete the treat t with the application of a paint jm that possesses adequate ex )r durability, and is highly re int to moisture. As such a pro ; will be mainly used on rougher aces, it is also essential that the ective coating to follow should )f sufficient film thickness, erropro Rust Converter should be lied only over firm rust but not :lean metal, because some excess ity would remain on the surface :ss this was carefully washed off r to painting.

Protective coating

. painting scheme, to be effective, it be based on a primer pigmented i substances capable of inhibit corrosion. The inhibiting sub ce can operate to the full only if surface preparation is well done, igments that usually show in tive qualities include red lead, te lead, lead chromate, basic lead •hate, zinc chromate, and zinc le. The value of calcium nbate, lead silicate and lead namide has recently been claimed, hin the British Iron and Steel earch Association, numerous prim paints have been tested, several which have proved superior to well-known red lead in linseed priming paint. he value of these primers certainly

has been proved where they have been applied to well cleaned and en tirely rust-free surfaces. Yet there was a need, for some considerable time, to produce a primer with such specific features as: • To be applicable to surfaces which are not entirely rust free and, in particular, light pitted rust. • To dry quickly, to have good adhesion, abrasion resistance, yet to remain flexible. • Should not interfere with weld ing operations and fabrication. • Should not generate toxic fumes during welding. • Should be compatible with all types of protective coatings, including epoxies and enamels. • Should be economical. • To be applicable in humid con ditions, to be able to replace moisture condensation contained in the pores of the metal.

Plastic emulsion primer

Ferro-Prime is a primer able to fulfil these requirements. In order to obtain desirable affinity to rusty metal, to give better penetration, etch ing and rust neutralising properties, this product has been formulated as a plastic emulsion primer. It forms, in one application, an inhibitive phos phate film and a priming coat. It is a synthetic based material which, on curing, becomes resistant to water, oil, certain hydrocarbons and diluted chemicals.

Prior to its application, it is neces sary to remove grease and loose rust, also to rinse the surface with fresh water where contamination is suspected. From various reports from sections of industry, it has been found that it is possible to carry out in one day a satisfactory repainting schedule, consisting of the applica tion of Ferro-Prime, the approved undercoat and finishing coat. There is, as yet, no classification for this type of primer, and its properties do not fit existing specifications.

The idea of corrosion protection, especially the protection of farm machinery is not always well accepted.

Recently it was suggested to a firm making outdoor machinery that the life of many of their paint coat ings with a consequent reduction in rust formation could be improved by more attention to the preparation of the metal. The reply was that they expect only a limited life from their machine (not more than five years) and it would be a waste of money to indulge in more sophisticated and

expensive surface preparation. Is this attitude right? Surely not!

Admittedly it is difficult to work up much enthusiasm for the preven tion of rust on a seeder that is used for only three to four weeks out of 52, and for the remainder of the time is left out in the wet and dirt.

It is bound to rust, and since the metal for strength reasons, is well able to withstand five years rusting— why worry?

Further, when it gets rusty it will not offend the aesthetic senses of most of its users; but aren’t bolts more likely to fail, rust to get in the bearings and the mechanism to be sooner clogged, if it is not pro perly painted and otherwise cleaned and protected? Thus maintenance costs are increased, even if the seeder is traded-in every ten years, which is unlikely.

Corrosion costs money, in main tenance, loss of production time, and in the necessity of too early replace ment of machinery or roofs of build ing, storage tanks and other structures.