Process for the production of galvanized steel sheet protected by chromium and chromium oxide layers

Improved process for the production of coated steel sheet particularly suitable for the fabrication of car bodies, galvanized and further protected by a layer of metallic chromium and hydrated oxides of chromium. The improvement consists in optimizing process operating conditions, within certain limits, to obtain a protective layer of chromium and hydrated oxides of chromium with absolutely new morphological characteristics which endow the product with corrosion resistance far greater than that of similar products reported in the literature.

The present invention relates to an improved process for the production of 
coated steel sheet. More precisely, it concerns the optimization of 
process operating conditions, within certain limits, for depositing on 
galvanized steel sheet a further protective layer of metallic chromium and 
hydrated oxides of chromium with absolutely new morphological 
characteristics, which endow the product with far greater corrosion 
resistance than that of similar products reported in the literature. 
Processes for obtaining similar products have already been described in the 
literature, for instance in French Pat. No. 2,053,038, British Pat. No. 
1,331,844 and Japanese Pat. No. 47-29233; the corrosion characteristics of 
the products obtained by the processes described in these patent 
disclosures and confirmed by tests made via specific examinations during 
the research work that has led to the present invention, are good, but 
nevertheless they still do not meet the current needs for car body makers, 
which are very demanding in some cases. 
For instance, British Pat. No. 1,331,844 describes a product consisting of 
galvanized sheet that is further protected with a layer of chromium and 
chromium oxide. Painted, scratched testpieces of this product subjected to 
the salt-spray (fog) chamber test as per the ASTM method show signs of 
white rust and traces of oxidation of the ferrous substrate after 1850 
hours, while unpainted testpieces under standard conditions reveal signs 
of rust after twenty-five hours. These findings are confirmed by tests we 
have run on products obtained by us experimentally according to this 
British patent. 
Although such products comprise a considerable step ahead compared with 
conventional galvanized or paint-protected strip sheet, they have not been 
manufactured commercially both because of their cost and because they were 
considered less advantageous than prepainted products, about which 
however, there have since been second thoughts. 
Furthermore, for some applications, such as the lower parts of car bodies, 
particularly exposed to the deleterious effect of trapped moisture and the 
salt increasingly used to keep roads ice-free, the quality of the 
galvanized products protected by chromium and oxides of chromium as per 
the present state of the art still appears unsatisfactory. 
The need for further protection of galvanized sheet stems essentially from 
two facts: the corrosion products of the zinc, which is sacrificial 
vis-a-vis the ferrous substrate, are incoherent, thus causing the 
breakaway of the overlying film of paint; secondly, where aeration is poor 
at a mixed-material joint such as the joint between coated and uncoated 
steel sheets, or in the vicinity of scratches, the zinc-iron galvanic 
couple beneath the paint causes local alkalinization that saponifies the 
paint which peels away, thus aggravating the damage. 
These drawbacks are avoided by covering the zinc deposit with chromium; but 
for cost reasons, the chromium deposit is extremely thin and under the 
known deposition conditions it occurs in the form of relatively large 
particles, with average dimensions around 0.1 micron, which leave 
relatively large areas of zinc uncovered. 
The purpose of the further layer of chromium oxides is to cover both the 
chromium and these bare patches. However, still within the ambit of known 
depositional conditions, this layer of chromium oxides is sometimes 
incoherent and discontinuous, and especially fairly soluble in alkalis; 
therefore, if mixed-material joint conditions occur with the consequent 
alkalinization of the ambient, this additional protective layer is not 
very effective. 
The object of the present invention is to eliminate these difficulties by 
providing optimum process conditions which make it possible to obtain 
galvanized sheet further protected by a superimposed coating of chromium 
and hydrated oxides of chromium, containing only a limited total quantity 
of chromium, thus keeping costs reasonable, the morphology of this layer 
of chromium and oxides of chromium being such as to ensure better 
corrosion resistance than that of similar coatings described in the 
literature. 
According to this invention the improved process for depositing a 
protective layer of metallic chromium and oxides of chromium on a 
galvanized steel sheet is characterized by the following sequence of 
stages: 
Continuously dipping the galvanized steel sheet in an aqueous solution 
containing from 110 to 170 g/l CrO.sub.4.sup.2- ions, from 0.7 to 1.4 g/l 
SO.sub.4.sup.2- ions, from 0.4 to 1 g/l Cr.sup.3+ ions, from 0.5 to 1.1 
g/l F.sup.- ions and from 0.01 to 2 g/l BF.sub.4.sup.- ions, the 
solution being held at a temperature of between 40 and 55.degree. C. and a 
pH of between 0.3 and 1, 
Maintaining a relative velocity of more than 0.5 m/s, preferably between 1 
m/s and 3 m/s, between the sheet and the solution, 
Imposing a cathodic current density of between 40 and 80 A/dm.sup.2 on the 
sheet for a time between 2 and 6 seconds, 
Extracting the sheet from said bath, eliminating the maximum possible of 
the adhering solution, 
Continuously dipping the sheet thus obtained in a second aqueous solution 
containing from 33 to 52 g/l CrO.sub.4.sup.2- ions, from 0.4 to 1 g/l 
Cr.sup.3+ ions, from 0.6 to 1.6 g/l SO.sub.4.sup.2- ions, from 0.5 to 
1.1 g/l F.sup.- ions and from 0.01 to 2 g/l BF.sub.4.sup.- ions, the 
solution being held at a temperature of between 20.degree. and 35.degree. 
C. and a pH between 3 and 4.5, 
Maintaining a relative velocity of more than 0.5 m/s, preferably between 
0.5 and 2 m/s, between the sheet and the solution, 
Imposing a cathodic current density of between 10 and 25 A/dm.sup.2 on the 
sheet for a time between 5 and 20 seconds, and 
Extracting, rinsing and drying the sheet. 
The substances in solution are given in terms of ions participating in the 
reaction and not as compounds, since costs and availability of suitable 
chemical compounds can vary considerably from place to place and from time 
to time; in this way the cost of the solutions can be kept to a minimum 
without being tied to a rigid formula. Other ions are, of course, present 
in the solutions but these play no specific role and so they are not 
mentioned. 
With the restrictive operating conditions indicated above, a product having 
exceptionally good corrosion resistance is obtained. 
The zinc-coated sheet thus treated has an outer protective layer containing 
from 0.2 to 1.0 g/m.sup.2 total chromium, typically from 0.4 to 0.6 
g/m.sup.2, with between 80 and 90% metallic chromium, the remainder being 
in the form of the chromium in the oxides. 
The excellent corrosion resistance properties are attributable to the fact 
that, under the above process conditions, the metallic chromium is 
deposited as very fine discrete, crystalline, superposed particles having 
average gaussian dimensions of around 0.03 micron, at least 40% by volume 
of the metallic chromium being in the form of particles having a maximum 
size of less than 0.02 micron. The maximum statistical particle size is 
about 0.07-0.08 micron. In this manner almost perfect coverage of the zinc 
is ensured, since the average size of the areas that remain uncoated is 
less than 0.02 micron, while the total area of zinc remaining uncoated is 
less than 0.1% of the total area. This value has been ascertained by 
inspection under a transmission electron microscope of the metallic 
chromium layer detached from the zinc substrate. No breaks in the coating 
are to be seen at a magnification of 60,000 times. 
The layer of chromium oxides, deposited in colloidal amorphous form, plays 
an important role in ensuring the corrosion resistance of the product. 
This is because the colloidal layer provides almost perfect coverage of 
the whole surface of the strip and is present also in the very small zones 
hidden by the edges of the metallic chromium particles. There is also the 
fact that a short time after the treatment has been completed, this layer 
of chromium oxides becomes virtually insoluble in water and alkalis and 
only very slightly soluble in acids. 
The exact nature of this deposit is still unknown because the quantity 
involved is so small that it cannot be fully characterized chemically, 
while as it is amorphous, physical methods of analysis such as X-ray 
diffraction cannot be applied. Physical methods of chemical 
micro-analysis, such as micro-probes and the like are equally inapplicable 
due to the thinness of the deposit, which results in there being 
interference from the underlying layers. However, the layer contains 
non-metallic chromium and, considering the fact that it is insoluble in 
water and alkalis and only very slightly soluble in acids, it is assumed 
that it consists essentially of a partially-hydrated form of Cr.sub.2 
O.sub.3. 
The product obtained as per the improved process that is the subject of the 
present invention is endowed with excellent corrosion resistance, as 
already mentioned. A series of testpieces--unpainted, paint and 
X-scratched, and painted and deep-drawn (Eriksen)--were salt-spray tested 
(5% NaCl) in the fog chamber as per the ASTM B 117 method. On 5% of the 
unpainted testpieces, the first rust marks appeared after 900 hours, on 
20% after 1200 hours, while after 1500 hours 40% of them still showed no 
sign of rust. In the case of the cataphoreticallypainted test pieces with 
an X-scratch or deep drawn, there was no trace of rusting even after 2000 
hours. There was virtually no lifting of the paint at the edges of the 
scratches, while in areas farther away there was no blistering. 
Comparative tests performed by the ASTM B 117 method, using sheet treated 
as per known processes showed that the unpainted testpieces began to rust 
after between 20 and 100 hours, while the painted, scratched testpieces 
revealed traces of rusting after 800- 1800 hours, as well as frequent, 
small paint blisters. 
Electrochemical tests of galvanic coupling between sheets coated as per the 
present invention and bare steel sheets have shown this to be virtually 
nonexistent, thus signifying that the problem of the mixed-material joint 
has been practically eliminated.